It wasn't a withdrawable loan; the attacker used pumped-up MobileCoin as collateral to go leveraged-long on some other shitcoin in a wash trade against another FTX account which he (presumably) also controlled. The attacker withdrew the profits from the second account; when the first account went into liquidation MobileCoin tanked and the collateral was worthless.

At least that's what Michael Lewis' book Going Infinite says happened.

FTX's mistake was that their liquidation engine valued collateral using something that could be manipulated. It's actually a very hard problem for an illiquid asset -- the correct valuation is the order book depth during the margin account's liquidation -- but that's always in the future so you can't know it exactly. And an attacker with two accounts can manipulate most of the things you might want to use as an approximation (like the last trade price or the order book depth right now).

The book claims that the bankrupcy trustee found the exploiter and the trustee CEO claimed they "had video of him entering and leaving his house" in (IIRC) Mauritius. However since the funds haven't been recovered the bankruptcy trustee could be bluffing or simply wrong about this. Or they can't extradite the guy. So not quite total silence, but also definitely not resolved.

BTW the book is really good, although it feels a bit rushed (obviously Lewis had a hard deadline for optimal publication date so this is totally forgiveable). There is a crapton of juicy stuff in there, like "Operation Warm Blanket". The number of powerful people who have egg on their face because of that book is absolutely unbelievable. No redactions. So many of them would have been able to quietly sneak out the back door with no public record if not for Lewis. Oh yeah and the mother of all easter eggs on the inside of the dust jacket.