Of course there was also no rule saying that chess functioned on a restrictive rules system and a lot of evidence that it instead functioned on a permissive system. Which would invalidate the "no rule says I can't" argument.

(when designing a game's rules, "restrictive" means the approach is "everything is permitted by default, unless explicitly restricted", while "permissive" means "everything is forbidden by default unless explicitly permitted" -- if we assume that chess is a restrictive-rules game, we can similarly mate in one from almost any position by simply inventing new pieces with unusual abilities and declaring that "the rules don't say you can't!")

So, promoting a pawn in chess is undefined behavior?