It’s far from cut and dry, and there are multiple schools of thought and conflicting interpretations. Steven Hijmans is a revisionist. Traditionally, it’s indeed believed that it was originally a pagan holiday in celebration of Sol Invictus.
Steven Hijmans presents an alternative interpretation of the various texts that challenges this understanding. He may or may not be correct, but nevertheless, one should not assume that his views reflect a consensus among historians.
I don’t think you’ve paid enough attention. Back when ChatGPT first launched, they were treated as saints.
The negative opinions have corresponded with public sentiment souring towards them in general (this did happen quite quickly, however).