...doesn't that mean we can do X-ball, frisbee type sports for cardio purposes, but still need gym/body-weight training to maintain hunter-gatherer levels of mobility, bone density, and strength assuming we have desk jobs?

If we are fat and beginners, not. My point is that if a fat and weak person throws big hammy arms around with the frisbee, he or she gains quites some muscle. In the longer run, when it became a habit, yes, but that is a different story, I am trying to help people who never start or start and stop, not those who reliably do it and want to improve further.

Funny anecdote: I have seen some people in Eastern Europe call this half-seriously as "peasant body building", claiming that former peasant cultures really had this idea that be fat, because then you will almost automatically be strong in those circumstances. Because working all day in the fields is low-intensity cardio, with moving large hammy arms and carrying a heavy body around that actually builds strength, and then he can carry sacks of grain.

Of course I am not telling anyone to be fat. I am just telling if we are already fat that frisbee-type fun stuff is muscle-building in the beginning.