so did they lose this coin through a simple address collision or what?
After all these exchange "hacks", an address collision?
It's pretty clear that Bitfinex had a very insecure security implementation. One of their 2-of-3 multisig keys was on an online server -- okay, that's understandable because they need automated withdrawals. But apparently, with only the use of an API key (whose credentials may or may not have been located on the same, or another online server) and Bitfinex's hot key, BitGo would sign off on any transaction, no matter the size or depletion of reserves.
BitGo really dropped the ball here. They co-signed every single fraudulent transaction. That means they don't locally enforce any security flags or limits whatsoever -- they just take the first signer's word for it. That's a complete misuse of multi-sig. I hope BitGo burns for this.