According to a writeup at https://www.chainalysis.com/blog/lessons-from-the-resolv-hac... this started with a plain old hack that compromised their signing key.

They also had a smart contract which didn't do some proper checks, but the hack was only possible with the stolen private key. Whoever held the private key was able to mint a lot of money, unchecked.

So there was a traditional hack at the core of this heist, not just a smart contract exploit.