A "backdoor in bitcoin's hashing algorithm" would not help them recover a private key. "bitcoin's hashing algorithm" is, for PoW, SHA256. The only relevant break for PoW would be a break in preimage resistance; this would allow the attacker to mine blocks faster, which does not allow them to calculate private keys. They could use that to mine an alternate history where the ransomware attack did not occur, but that would of course be immediately obvious.
Preimage attacks tend to be much more rare than collision attacks. MD5 for example still has no publicly known practical preimage attacks.
Preimage attacks tend to be much more rare than collision attacks. MD5 for example still has no publicly known practical preimage attacks.