Therefore, if the quoted above is true, the attacker mined an entirely different fork, up to block 2500ish, by HIMSELF, allowing him to own the coins from the premine.
This entirely different chain had the most combined difficulty, allowing it to take over the current one (which had a very low diff.)
This allowed him to have control over all the 50M coins, and therefore deposit coins, reverse transactions (see quote #1), and so on. This is pretty much the only possibility IMO.
That's what I think happened. It wouldn't have been too difficult to do with the low PoW difficulty of the coin. Someone with a decent mining rig could probably have done it in a few days.