"Fox had little experience with game development; he and his three brothers often used RPG Maker 2000 to make role-playing games, though few were ever completed."

Fox still has all the assets for Undertale. All he has to do is port the battle system to Unity or something and then drop in the old assets. Someone already did it for the Sans battle: https://jcw87.github.io/c2-sans-fight/

The problem is, Fox is an unexperienced developer and doesn't know how to do that. He likely wants a one-click "convert my gamemaker game to nintendo" solution not an actual porting effort which would require rewriting parts of the game in a new language/engine.

> Fox still has all the assets for Undertale. All he has to do is port the battle system to Unity or something and then drop in the old assets.

Porting the whole thing over is a lot of work, speaking as someone who has actually worked on trying to port a game away from Game Maker.

> The problem is, Fox is an unexperienced developer and doesn't know how to do that. He likely wants a one-click "convert my gamemaker game to nintendo" solution not an actual porting effort which would require rewriting parts of the game in a new language/engine.

No, he just doesn't want to waste his time on reimplementing the entire game just to run on one more platform.