Read the whole thing again. They aren’t complaining about the 15,625 shares (which they presumably sold a long time ago) but the remaining 25,000–15,625.

"NVIDIA’s CFO wrote me a letter stating that 15,625 shares of my stock options had vested, and that I was required to exercise them. I did, and then forgot all about it."

Normally this is a right to buy at a given price. If he was billed for it, then there is very little chance NVIDIA can weasel they way out ot it. I suspect he never paid for the shares.

Those shares were fine, it was the other 9,375 that were in question here.