This is not just some N64 emulator running the game, but rather a real PC port of Super Mario 64. This was made possible due to someone decompiling the game last year to give developers full access to the source code, which can still be found on the Github repository. One Nintendo fan used this code to create a PC port, which runs on DirectX12, that can run in widescreen 4K resolution. Read more for a video and additional information.
Rather than officially announcing the DX12 Super Mario 64 port and releasing a single level demo, the creator uploaded the entire game online, leaving fans having to scour the internet to find the download links. This was done so Nintendo doesn’t shut the entire project down before multiple outlets are able to offer links to the game. In other news, a Nintendo Switch port of this game is rumored to be on the slate for later this year.