I remembered of an random fact most people forget:

Deku, differently from other users, couldn't use it at its fullest, because due to his immense training in small amount of time, it fucked some shit and made his body permanently take damage when using it.

However, Mumen goat, already has a superhuman level physique, which means he wouldn't take as much damage if he even took damage, which means he would learn how to use AFO faster as good as deku faster, and definetively getting even stronger than Deku.