The game has already sold millions before Switch release, and was the best selling game (not counting gamepass) for few months when it was released.

That being said, I have no actual proof that those people weren't Switch 2 owners who just bought it on other platform because it wasn't out on Switch 2 yet.

Yes, but don't we all buy games over and over again? But also, there's probably a decently sized loyal Nintendo group who are seeing some of these old games for the first time.