I don't agree with this at all. I think someone who is good at breakdancing would have a significant advantage even as a beginner in BJJ. It would seem to me that the strength, coordination, body awareness, and rhythm a break dancer develops would be very complimentary to BJJ. And conversely, training in BJJ could help a person become a better breakdancer.
Some things are general and universal or nearly so: muscular strength is applicable to almost all physical activity, this is also true of flexibility with the exception that more strength is almost always beneficial where more flexibility than the max needed is not always beneficial. Conditioning and metabolic adaptations are more specific, there is good research to show that there is very little shared benefit to doing conditioning work for two wildly different activities. The easy and extreme example is that doing ultra marathons will have essentially no benefit to your endurance for 3 minute rounds of a combat sport, but this is still true, to a lesser extent, if you use hiking in the mountains and sport fencing as your example activities. The hiking
might improve your leg strength, which would benefit the fencing, but is unlikely to improve your metabolic, or "cardio" fitness for fencing.
Then there is skill transfer and this is complicated. I used the term "natural athlete" to describe those people who are able to easily learn one physical skill and then apply the lessons learned to other physical skills. Using that term was apparently a poor choice as it has caused a lot of confusion. These people may have terrible coordination, asthma, poor genetics for developing strength, etc., etc., that make them terrible at athletics, they're just able to apply lessons learned in one physical activity to another easily. The research shows that these people represent a sizable minority of the general populace. I don't remember the details, I'm guessing something like 1/3-ish of the population is able to do this and frequently those people we think of as natural athletes, who can pick up all sports well without seeming to need much training, do have this ability along with other physical gifts, but there are many pro athletes who do not learn this way.
Most (small majority) people don't transfer skills well between different activities beyond fairly basic levels and if the activities are very similar without being the same they will tend to confuse each other at a high level of performance or competition. Even those for whom skill transfer between different activities comes easily may suffer some skill confusion when performing similar but not identical activities at a high level.
If someone learns to play the trumpet they will be able to pick up and play the french horn at a basic level because the basics are just about the same, regardless of which type of learner they are. If they are a very skilled trumpet player
and someone who is able to transfer skills between activities easily they will probably be able to pick up the french horn and play it at a pretty high level almost immediately. If they were auditioning for first chair trumpet with the London Symphony and they spend their time playing the french horn in preparation their performance is unlikely to be as good as it could have been if they'd spent the time playing the trumpet and possibly worse than if they hadn't played anything at all. If they did not transfer skills well between different activities then they are guaranteed to do worse by practicing the french horn than if they had spent the time practicing the trumpet and likely worse than if they'd done no practice at all.
So, to talk about break dancing and BJJ, if someone who regularly practiced break dancing took up BJJ, I would agree that they'd be much better off than the couch potato doing the same thing. To the extent that their break dancing had developed muscular strength that would be entirely or almost entirely transferable to BJJ. I've never done break dancing, but to the degree that the intensity, duration, etc., of break dancing is similar to BJJ some measure of metabolic adaptions/conditioning would also likely transfer. If there is overlap in the kinds of flexibility needed for both activities those would also apply. If there are skills that overlap, like say learning to fall, then those would transfer to some extent but the differences (being taken down rather than throwing yourself down for example) would likely cause some initial confusion for the student that does not transfer skills well and might continue to do so if they switch back and forth between the activities. If this person was the type that transfers skills easily between different activities then they might gain some other insights that they could apply as well. Since these are pretty different activities they are unlikely to be overly detrimental to each other. The student is unlikely to inadvertently try a break dancing move in the middle of a BJJ tournament by mistake, but their falls might not be as safe or theatrical as they would have been if they specialized.
MMA is also tricky because it's got a lot of components that need to be mastered or at least addressed. Is someone's MMA hurt by doing a lot of BJJ tournaments? To some extent, maybe? How much do they get used to relying on techniques that are safe within the BJJ rule set and will get you knocked out or injured in MMA? Do they have enough training time to get maximum benefit for their non-grappling MMA skills? Will learning boxing hurt their MMA? Again, probably only to the degree that the difference in rules could lead to some unconscious mistakes and to the extent that it limits their time for training kicks and grappling. Will learning boxing and BJJ improve their MMA? Almost certainly because those skills, or similar skills, are integral to MMA.
Now, would boxing help your tournament BJJ? Well, the conditioning and strength requirements have a fair amount of overlap, so that would transfer for everyone. To whatever extent the boxing clinch work was applicable it might transfer to a greater or lesser degree depending on your learning style. I have no idea how things like resistance to fear or pain might or might not transfer, but that might be a benefit regardless of what kind of learner you are. But if you are in the majority of people who do not transfer skills well between different activities you are unlikely to get much of any kind of insight into how to apply groundwork skills better because you box.
An example of the confusion I'm talking about: I had a friend in college who had done a lot of Muay Thai and after his MT school closed he decided to take up TKD. He had a really hard time in competitions because he would unconsciously sweep his opponent's legs and punch them in the face, neither of which was allowed within the rule set of the TKD competitions he was doing. He gave up on TKD because he felt that for the way he learned things he would have to develop things he considered to be bad habits to be able to compete.
None of this is to say that you shouldn't train in whatever you want or that interests you. And this goes triple if your goal isn't to be a professional athlete or something like that. I think that if you've got 2 hours/week you can spend on martial arts training and you want to be competent at BJJ you should probably spend 2 hours/week on BJJ and not 1 hour on BJJ and 1 on boxing unless they're both equally important to you and you don't care how long it takes to become competent. Outside of time restrictions, I don't see any reason for the non-professional to limit their activities. I think it's much more important to enjoy what you do than to over optimize all the fun out of it. I just think it's a waste of time to take up water polo because you think it'll make you a better cellist.
