IMDb isn’t the most reliable, but American Cinematographer normally is. https://www.theasc.com/magazine/april03/cover/index.html

The X2 article (ASC’s online archives, for some odd reason, skip between 1999-2001) refers to the first film being shot in anamorphic before explaining why X2 was not. If anyone can find an article from the AC July 2000 issue, it should completely confirm this (and possibly mention whether any shots, like VFX shots, were done in Super 35 or VistaVision-- as the “Moulin Rogue!” article mentioned one scene was shot in Super 35 for VFX reasons-- but at the same time those shots could be hard-matted. Given that “X-Men” did not go through a DI, any 4x3 transfer would likely be sourced from an anamorphic IP).