I think most of it comes from the fact that for a Marth player, Wario's options are still opaque and novel, whereas a P:M Wario player who has at least a basic understanding of the character will have a deeper understanding of Marth's options, which haven't changed much at all from Melee, and how to exploit his weaknesses, and from there you just ask yourself which of Wario's moves fit into those roles.
Basically, JC knows more about Wario than P:M Marth mains, and he knows just as much about Marth.
Basically, JC knows more about Wario than P:M Marth mains, and he knows just as much about Marth.