I think it's pretty accurate to say that to be a top MMA competitor you've got to be in pretty amazing shape, and that it's also true that in most cases you aren't going to get through a real military boot camp without being in some sort of reasonable shape. This doesn't necessarily make either of these, or other forms of "hard core" martial arts or military training the best, fastest, safest, or most efficient way to get in shape for a specific individual or for a specific goal.
Military training is definitely not the most efficient way for an individual to get in shape. It's an efficient way for governments to get large numbers of young people in adequate shape for the work they'll be called upon to do, while at the same time instilling a necessary response to their superiors, mental toughness and indoctrination into military organization - all while providing basic skills training with weaponry, etc. It's designed to work well enough, fast enough, for enough young people to become fit enough, without washing out an excessive number of potential service members.
Now elite units may get much more training, but even then it's focused on the job role, not the individual. And with elite units especially, survivor bias (
Survivorship bias - Wikipedia) plays an enormous role in appearances. You don't get to be a US Navy SEAL if you can't hack the training, so if you are a SEAL you're going to be in pretty great shape, people who are incapable of doing the training don't get to be SEAL's. It also means that if you have the potential to make it as a SEAL you're likely to get great results from lots of different training programs that only produce mediocre or poor results for other people who couldn't make it as SEAL's.
Survivor bias is also huge in any pro sport, be it pro level MMA, soccer, basketball, Olympic champion, whatever. If you can make it at that level not only have you trained really hard, you are gifted with tremendous genetics for that activity. This presents in a lot of different ways, some of them obvious and some less so. For the obvious; You don't get to play in the NBA if you're 5'5" pretty much no matter how skilled you are at basketball. At the same time, at the elite level that is the NBA, you still don't get to play if you're 7' 2" and aren't amazing at basketball. Going further, it's obvious to pretty much everyone that no matter how much you play basketball and how early you start it isn't going to make you taller, though it'll probably give you better wind and stronger legs.
Strangely, people can't seem to figure this out when it comes to most other athletic activities. I regularly hear people say things like, "I want to have the long, lean body of an Olympic swimmer so I'm going to swim". And sure, swimming can help with that lean part as could basketball or MMA, but Olympic swimmers mainly all have the same kind of body because that's the best body type for an Olympic swimmer and just like basketball, people who don't have that body type don't make it to the Olympics for swimming. The most frustrating for me personally is when people tell me they don't want to lift weights or do resistance training because they don't want to look like Mr. Olympia. As if looking anything even vaguely like Mr. Olympia didn't take an extraordinary combination of very unusual genes, tremendous work, and an exotic cocktail of steroids and hormone use.
Beyond dictating how well structured your body is for an individual sport, genetics, along with overall health, age, stress levels and similar factors play a big role in how you're going to respond to training. Having worked as a strength training instructor for over a decade now, I've seen people who can recover quickly and get fantastic results doing very high intensity strength training 2 or more times a week, along with intense martial arts or other athletic activity on top of it. I've also seen clients who make no progress or regress with that sort of regimen, but get good results doing only one or two high intensity workouts per month combined with regular low to moderate intensity activities. Some of the first group would probably make it as SEAL's where none of the second group would, but that doesn't mean that most of the second group can't become much stronger and fitter than the average hobbyist athlete, they just have to train in a way that works for their current life situation.
Finally, fitness is fairly specific to individual activities. If you want to be able to do 5 minute MMA rounds you need to train very differently than if you want to run marathons. Both of which are different than the ideal choices for a competitive power lifter. We all have limited recovery systems and a limited tolerance for work so we can't train optimally for all things. Even if you have the genetics to be both an Olympic power lifter and a Navy SEAL you probably can't do both at the same time and almost certainly won't be as good at either if you can and do both. Luckily, if you're in great shape for one form of athletic activity that should be sufficient for good health and there are few situations where being in shape for one activity is actively detrimental to another.
Sorry for the gigantic post. I'll end here...