Probably because it's based entirely on (non-black-box) reverse-engineered Switch binaries, and is therefore very possibly a derivative work of Nintendo's copyrighted code.

To not get sued six ways from Sunday? If a company could sue me out of existence, I may be nicer than I have to be to them.

Literally nothing I have ever read about Nintendo makes me think this would even give them a moment of pause as they sue the author into oblivion. (This is a comment on Nintendo's behavior, not the legality of the author's actions.)