There is no rule in terms of the actual SSB games. What was said was that if youre going to create the "rule" or THEORY that a character is deconfirmed by a costume that represents an unconfirmed character found on a confirmed character, then you must apply it to every situation it is revelant regardless of your personal bias.
Example: If Wolf Costume found on Fox then Wolf deconfirmed. So If Waluigi Costume found on Wario then Waluigi deconfirmed. You can't say "Oh I like Waluigi so that cannot be the case." Personal bias cannot change the rule.