Nintendo abused the DMCA system to take down videos of Switch emulation before. They've hired private investigators to go after and intimidate modders.

Their legal team knows that with their excessive legal funds they don't need to care about what the law allows them to do, their lawyers and henchmen can simply bleed their targets dry and make them give up.

They're a terrible company that happens to make some fun games.

Nintendo is still a multi-billion dollar company, they can afford to sue him and burn through his time and money even when they know they're wrong.