FTX didn't have any coins to sell, it was all just debt, so it couldn't have affected the market that much, unlike Terra for example who had to liquidate their coins in that botched bitcoin-backed stablecoin. Grayscale in this scenario of dumping would probably crash the piece even if would spread it over weeks, it's 10 billion worth of coins with almost no buying power, with every company in debt and everyone seeking loans and a dump would just trigger more bankruptcies, and more money of what is still left fleeing to other "safe" assets.
That being said, Grayscale has zero reasons to sell their coins at this point, absolutely none.
There is another hypothesis, and that's actually the dangerous one because it has declined to publish its proof-of-reserves there is speculation that they actually don't have that many coins. Of course, it would mean fewer coins to dump but also another clusterfuck that would bring down other companies and trigger bankruptcies.