The origin is really unclear. It's not certain which id employee added it to Quake 3 and where they got it from. John claims it wasn't him. Gary Tarolli, one of the founders of 3dfx, claims to remember tweaking the value of the hex constant to the value known today. But he says he didn't come up with the algorithm either. Maybe somebody else at 3dfx did, maybe he got it from somewhere else.
And at 3dfx he probably learned it from some ex-SGI guy, and that guy learned it doing his PhD at Stanford from someone who worked for ed Ed Catmull and so on. The lore goes deep with stuff like this. There's rarely just one author but many who improve the formula over time.
there's an unrecognized genius out there that realized you can just apply a bit flip to get that out there. That John Carmack hasn't claimed it as his invention when he could have, speaks volumes about his character.
Except he couldn't have even if he wanted to. There's always the risk some former 3dfx or SGI graybeard comes out with some paper binder with the original implementation, and calls you out on your bullshit if you do. When you're such a famous public figure no way you ever risk lying in public. Even if he were to getaway with it there was nothing for him to gain: he already has all the fame and money from honest work, there's no point risking it all to claim some piece of code.