It comes from the fact that he's had plenty of tournament experience, which allowed him to improve his own skills. Although he didn't exactly play Little Mac, he was consistently playing with top players and got to experience the variety of playstyles that his opponents had, which helped him adjust his own playstyle and his ability to read and punish his opponents for the mistakes they do. Not to forget that consistent practice and attending tournaments can help develop your mental game and focus during matches so that you don't autopilot/get nervous often during the match. He's already used to a tournament-like atmosphere, so he knows how players react in certain scenarios.
I do feel like he could have a lot more potential if he continues to practice with the character offline, since online possesses a different metagame than offline. Little Mac is buffed online because of the fact that it's a lot harder to react to his moves, with his KO Punch being extremely difficult to react to. Not to forget that he's a fast character on the ground, which can be difficult to deal with because of the input delay.
