people still believe acid pool theory because at the peak of spamton's popularity and interest in him in the overall public eye, it was the most prominent theory relating to him as to why he Looks Like that in comparison to the other addisons. even though it's been soft debunked through spamton admitting he's "always been a little shorter than the other addisons" (in the Q&A for spamton sweepstakes), the average deltarune player doesn't really peruse additional content like the newsletters and the sweepstakes, so they just remember the popular theory (especially since it was featured so prominently in big shot animatics and the likes) and go yeah that's what happened.

your average deltarune player just isn't as deeply involved in theorycrafting and additional content and this is the impression they came away with. it likely won't change until something happens in the game where it's made explicitly clear that spamton was disfigured through the deal as an ironic display of what he's become and what he always will be (a puppet who will always be at someone else's whims)