I don't buy that. The fight choreography is worked out in pre-production, its not like they are practicing it while on set with all the production team standing around. It does take time though, and if the principle actors want in on it then they gotta put in the work versus letting stuntmen do it all with wigs, quick cuts, or now CGI facial masking. The ONLY reason MCU films have generally terrible hand to hand fights is because it is a CHOICE to make them more CGI "let's handle it in post" versus "pre-production is when we get this right" physical stunt work. They want the actor to be able to do stuff so there are some physical stunts on set but nothing like what a dedicated martial arts film can do with a skilled practicioner as the lead and proper choreography. Shang-Chi had, for marvel, some pretty good fights but then it totally shit the bed with that third act. Otherwise Captain America films are about the only ones with decent physical fights that look like there is weight in the punches.
But if you watch an america action film from the 90's or before, we did know how to do physical fights, we just lost it somewhere along the way with jump cuts, shakey cam, and reliance on CGI.