I apologize that I've been too busy to get to my breakdown of what skills do and do not carry over from different forms of weapon combat sports to each other and to actual weapon fighting, but I thought these comments deserved a bit of a response in the meantime.
Whether they use the forms & techniques of HEMA or of the Asian arts, if the sword hits you, it hits you.
This raises a question which gets back to my original discussion of the "accuracy" of HEMA. Why do we need to bother with the historical manuals? Why can't we just do a bunch of sparring (LARP rules, SCA rules, HEMA, Kendo, Olympic fencing, whatever) and see what works?
The problem is two-fold. The first part is that sparring (like any training method) is a simulation which differs in various ways, large or small, from a real fight. The second is that actual sword fighting with real swords is pretty much gone from the world. Armies don't use swords on the battlefield. Gentlemen don't decide questions of honor by dueling with real swords. Sometimes in societies where machetes are common workplace tools, you'll find them used for assaults or murders, but not so much for regular dueling.
If you look at other aspects of fighting or martial arts, we have a lot more to go on.
Want to know how grappling works, under specific rulesets or in a street fight or in law enforcement or in a number of other scenarios? We have millions of people doing some sort of grappling every day and we have tons of data on how it plays out under different circumstances.
Want to know how to win a fist fight? We have countless hours of video with people getting knocked out by punches in boxing matches, karate matches, Muay Thai matches, MMA matches, street fights, bar fights, you name it. We can compare our training and our sparring to how those fights play out and even try it out for ourselves if we like.
Want to know how to win a full-contact stick fight, the sort where you may need to knock out or disable an opponent? That gets trickier and more dangerous. The Dog Brothers are probably the gold standard for that sort of training (although they aren't the only ones who do it). They don't spend the majority of their training in full-contact matches, but they do have them as a reality check for the rest of their training. They try to avoid permanent injury, but fighters do get knocked out or temporarily disabled. I'd say their experience gives a reasonably realistic picture of how actual stick fighting between skilled combatants can play out.
When it comes to swords, we just don't have that kind of first hand data. Swords are not sticks. Being able to tag your sparring partner first with a boffer weapon or a foil or a shinai or a feder (the most common form of steel training weapon in HEMA) does not necessarily translate into surviving a fight to the death between opponents with sharp swords. So we try to validate our sparring experiences against the historical record and our cutting practice and hope that by putting it all together we can have some approximation of the reality.
Have you been to Pennsic? It's mostly about 8-12 foot spear poking in a line battle. And I've never seen a HEMA battle of more than 30 people at once, let alone 50 to 100 to 500+ for some LARP events. My point being, you have no venue to train nor prove that you have the skills of fighting in a battle with your HEMA skills among other HEMO people. While in LARP & SCA, you can.
I have indeed been to Pennsic a number of times and have fought multiple times in the field battles, the woods battles, and the bridge battles. I've also participated in group melees at smaller events.
I do agree that one of the most valuable insights experience in the SCA brings to understanding medieval warfare is grokking the difference between one-on-one dueling and fights between mass formations. It's not so much that SCA mass melees are all that accurate in the exact specifics of historical battlefield formation fighting. (They're not, overall, although I'm sure there are some details which might match reality.) It's more just understanding that group fighting is different from individual fighting. Being able to maintain a tight formation, moving in sync with your comrades and protecting them as they protect you, is key. The clever evasive footwork you might rely on in an individual duel is no longer relevant.
For the record, most HEMA practitioners are working from sources which are focused on individual or small-scale combat rather than full-scale army battlefield tactics. I believe there are some extant treatises which address formation battlefield tactics, but we don't have the numbers to practice marching around in pike squares. Maybe at some point in the future we'll have large groups which will explore that sort of scenario.