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.
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
In a court of justice you'd be right, of course.
But for online armchair speculation, you have to admit it seems a likely explanation.
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.