UST wasn't a true stablecoin (it isn't like any dollars disappeared from the ecosystem), so I'd put it in the same category as the hundreds of billions in "vanished" market value that was a simple consequence of the declining value of crypto in general. Whether or not Alameda was holding a bunch of UST or Luna, is another question. SBF seemed to be pretty all in on fringe shitcoins, so I would not be surprised if they had significant exposure.