Sounds like if they had a checking account with their bank credentials stored in ENV variables and someone got access to that server it would be the same outcome.
The details of it being on a crypto-currency are interesting but when password/passphrase/private key security is poor it doesn't really matter the medium holding the money.
If the hackers are sophisticated, I would think they would start wiring in much smaller amounts and thru accounts so tracing is harder. Much like what they are going to have to do with the funds in that wallet.
If they setup some plausible 3rd party company the game studio could use and started transfers of $10k a pop it might be some time before anyone catches it.
Transferring $650 million out of a corporate bank account would usually require in-person approval by a C-level officer, or at the very least, prior notice to the bank of the transaction.
The details of it being on a crypto-currency are interesting but when password/passphrase/private key security is poor it doesn't really matter the medium holding the money.