This definitely has all the hallmarks of a:
"Hey what happens if i do this?"
"Ha, i just stole some of their Coins! How easy was that!"
"I wonder how much i could steal?"
"Hmmm. That's a lot of money. Umm how do I turn that into cash?"
"Oh man. Look everyone's talking about the theft, how awesome am I?"
"Ohhhh shiitttt! This is not good. Everyone is trying to track me down. Oh shit, maybe they can track me. And none of my attempts to turn it into cash have worked. Everything is getting blocked. And oh no they're publishing the wallet details. Everyone knows where the coins are. Oh shit!!!!!!!"
"Wait maybe if i give it back, and pretend I was just doing it to highlight their security failures, maybe People will stop looking for me... Shit, shit, shit.... Please stop looking for me...."
This does not have the hallmarks of a professional on it. If it had been a professional, the moment the money began to leave Poly, it would have been being split up, and sent to multiple places where it was being turned into actual currency, before anyone cottoned onto the fact a theft had happened at all. It would not have been sitting in a single account, just waiting to be tracked down.
Alternatively, if they stole $600 million, and Poly gets back say $599 million. A $ 1 million dollar pay-off is not too shabby for a nights work... And $1 million is a lot easier to hide than $600 million...