I really don’t think it works like that… If the AU belongs to someone else, how can someone who’s not the AU’s creator make a branch of that AU? That doesn’t even make sense.

How’s it any different from any other fanfiction, anon? I don’t suggest that the undertale anon in question DISTRIBUTE this branch AU in any way, if that would make the author uncomfortable, however if they feel that by it existing they have brought all of that Sans’ friends back, there’s no reason for them not to write it.