Except he didn't do that; he led by ramming his arm as far as he could into the tendons, shooting off a five-finger blast to shred the tendons away, before using a palm strike with what was left over to actually attack the (now vulnerable) Muscular. Deku wasn't getting stronger, Muscular was getting his tendons ripped away and incorrectly made an inverse read as a unreliable narrator (tl;dr Muscular was in fact getting weaker).