Yeah, it could be a case of syncretism: Pythagoras' school was perhaps a meeting of minds that took a bunch of disparate ideas/concepts from all over the world and finally nailed them down into more concrete shapes and merged similar but equivalent concepts into grander shared abstractions. Then things like "Pythagoras's school found" and "Pythagoreans believed" accidentally over the years get rubbed off to read "Pythagoras found" and "Pythagoras believed" in a "great man fallacy/conflation" between the school of thought and the nominal figurehead.
My understanding is that is possibly an incredibly common view on Greek history. For instance, I've heard that a lot of things attributed directly to Plato today seem better attributed to platonists in general and the wild maelstrom of activity in the forums.
Getting maybe further aside and into the weeds: Pythagoreanism was even a multiple-times revived religion in Greek culture, so that would give further reason to attribute things to Pythagoras as "religious texts" things from other scholars or things from later movements (such as neopythagoreanism, which I found in one rabbit hole deep dive has a lot of interesting but unproven ties to early Christian doctrine). (Similar to how so much scholarly work in Asian history got attributed to "The Buddha" before eventually some of them started attributing things maybe more accurately to just "a buddha".)