So are MMA fighters functionally isometric with boxers because they crosstrain boxing?
A boxer who would consider crossing over to MMA, if he wished to succeed at the game called MMA, would naturally have to readjust the proportions of training time he would normally have spent on things theretofore in preparation for the game of boxing. Which is to say, spending less of his limited time on boxing specific training, and more time with many other areas of interpersonal combat endeavor.
Just the same as with any other sort of athlete, be they boxer, thai boxer, catch wrestler, collegiate wrestler, judoka, jiujiteiro, or rugby lineman.
A boxer in MMA wouldn't box like a boxer, a freestyle wrestler in MMA wouldn't do takedowns like a freestyle wrestler, a thai fighter in MMA wouldn't clinch like a thai fighter, a bjj player wouldn't play guard in MMA like a bjj player, a sub grappler of any kind wouldn't do passes or submissions like he would pass or submit in his sport.
In pretty much any field of athletic endeavor, most especially more complex ones like combat sports, skills are too a great degree about 'what have you done for me lately'. Even if you grew up doing a certain thing, if it is not
maintained at the same level with the same levels of monthly investment up to then, then it will not avail you to that level.
In MMA, it is in fact rather straightforward to have great superiority in any one single area over just about any opponent, even ones who might have a 'background' in that area... but that is just the thing:
one single area. MMA is a cake with a lot of elements that can all interact in interesting ways. Not all have equal 'gravity' though, of course. That's the key thing. Identifying those vital
schwerpunkts, and, if you believe in them, if you
invest more into them in your preparations than an opponent of comprable talent as yourself, even if he 'grew up' doing things that may have involved them, if he doesn't invest as much as you have, then you will certainly overcome him there.
