Would assume that in this case that the gpu was created and assempled using a open blueprint/schematic thats freely available for anyone to see use and modify. Fairly common thing when it comes to stuff like 3d printing where many people freely share their models for others to use.
If you want to make a fan game, make spiritual sequel instead.
Nintendo can’t do much about games that are very similar to Mario, Zelda or Pokemon for as long as they do not copy any assets, (some) names or levels from those games. Based on the videos the main character is unique but most if not all the other art assets are direct copies from the original game.