Board Thread:General Discussion/@comment-32182236-20181101220519/@comment-31536324-20181104150936

Exactly. The one point I always see is that people keep trying to explain how Asriel is alive despite being stuck as Flowey in all endings of Undertale. Something that should have immediately keyed them to the alternate universe/parallel timeline idea.

Even then, the only way it could be a prequel in any sense is with the Sans and Papyrus' (and possibly Gaster) stuff, which isn't much of anything major to the overall story of Undertale anyway.