I'm just a 'regular security guy' but in that link you posted they detail that after the initial phishing compromise "The attacker managed to leverage that access to penetrate Sky Mavis IT infrastructure and gain access to the validator nodes." They don't detail the bugs that got them access to the nodes but this didn't give them control of the network so "the attacker found a backdoor through our gas-free RPC node, which they abused to get the signature for the Axie DAO validator. ...Sky Mavis requested help from the Axie DAO to distribute free transactions ... Axie DAO allowlisted Sky Mavis to sign various transactions on its behalf. This was discontinued in December 2021, but the allowlist access was not revoked."
Sounds like a pretty classic hack to me. They got into the network, got access to some important servers (how? they should be totally segregated from the corporate network). Then found a depreciated endpoint that allowed them blindly sign transactions. This is bread and butter for any pentesting work, makes me wonder if any of these web3 orgs are hiring security firms to test their systems and not just smart-contracts.