You can check for yourself though: https://research.facebook.com/people/research-area/augmented...
Going through that list regardless, I don't find Carmac there, and searching about it, seems he quit ~5 months ago. Might want to dig up a better example :)
And if you don't think one of the richest companies in the world can afford to hire whoever they want, then you're just being intentionally obtuse.
It's not that I believe they couldn't afford worlds best researchers, mostly questioning that the worlds best researchers would want to work at Facebook, no matter what the pay is.