[suddenclarity]

I had a quick look and apparently it started when he made a video on how to dump Switch games to ROMs and he showed 30 seconds of footage from Super Mario 3D World. Beyond that, almost all of his most recent video thumbnails contain emulators with Nintendo games on them.

The legal question aside, he's obviously trying to drive engagement by using Nintendo IP and it's no surprise that they eventually noticed him with his half a million subscribers. The risk taking surprises me considering what we know about Nintendo and that YouTube is awful do deal with. They even gave LTT a strike despite almost 16 million followers.