85% of the American male population is under 6' tall. Only about 10% of NFL players are under 6' tall. On top of that, people under 6' are more often than not better athletes than those over 6' because of the unnatural growth and joint difficulties that many tall people have.
That suggests that there is a MASSIVE talent pool of very athletic American men under 6' who are not playing NFL football. Especially considering that the athleticism necessary for the NFL and the athleticism necessary for flag football don't line up perfectly.
Do you think the Mexican players who did so well were bigger or smaller than them?
That's true. But if you stratify by position type, or if you use exclusion criteria that selects out front 7 and non eligible receivers, that number increases from 10%. Tyreek, Odell, Tank Dell, Jaylen Waddle, Derek Stingley, Xavier Worthy, etc are all guys who are under that 6' mark you talk about....and are highly likely to be more athletic than the guys on the flag team
Bruce Mapp, one of USA's top receivers (and also a DB) ran a 4.68 40 at about 6 feet, 207 lbs at his Pro Day. Pro Day times are notoriously faster than NFL Combine times.
The gulf in athleticism between USA flag players and the players the NFL could bring is tremendously large. I'm not even sure the team USA players would even get a chance to rip a flag because you can rip a flag only once the player becomes a runner. Tyreek would uncouple from a DB extremely easily and then once Murray puts that ball up no USA DB will be able to stop the completion.
As for the Mexicans: if they were smaller and were able to compete, that doesn't mean that if the NFL players fielded a taller team, that means they wouldn't be able to compete. It's like saying because AB is 5'10" and had a HOF type career, a 6'4" Randy Moss or 6'5" Calvin wouldn't be as good. Or because 5'8" Darrell Green was great, 6'3" Richard Sherman couldn't be great.