Where did Bill!Sans and Alastor!Sans come from?

Like, I can understand where Bill!Sans came from. Multiversal elements are already present in GF so bringing Bill into Undertale is just a fan being a fan. But Hazbin has no ties to a multiverse. Also, I feel like Sans would be way too smart to fall for Bill or Alastor's trickery. So how did those two AUs come to be?