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.
Usually I would expect proof for a positive - like that it was an inside job, or there being an indication of it. I'm not saying whether it was or not, just that it seems unusual for you to ask about proof of it NOT being an inside job.
When it comes to crypticurrencies, no, the "hack" that turns out to be an inside-job rugpull is so common that the correct burden of proof is on the people who think this wasn't an inside job.
Is there any proof that it was an outside job? If we don’t have any proof of either we should probably look at what is most common when it comes to crypto heists
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.