i believe it could be large blockchain data directories (if running full or archive nodes) such as deleted files, system files, application data and so on...
An Ethereum node alone is about 2 TB.
Even if there were wallets with all the coins on those servers, what is the probability that they were so unprotected and that some forensic expert managed to move them immediately?
Also, what is the probability that the eXch team did not have backups offline at another location?