Apparently [1], he managed to put them as collateral on a lending platform. That allowed him to borrow ETH. The way it works is that if you don't pay back the loaned ETH, the lenders can take the collateral. Except that here the collateral is itself frozen. So apparently, the ETH as now safe in the hacker's wallet and the lenders just lost a bunch of money.

I wonder why not swap them? And had he, would it have bricked the swap pool?

[1] https://x.com/czesmaill/status/2115988477905080716