Magical Tux probably doesn't have 500k BTC. Perhaps everyone on the site combined would add up to 500k BTC. I think every bitcoin on the site got liquidated. So he is backing it out. The problem is whether the coins got transferred out before he caught the transaction.
just as an aside, this doesn't quite feel right.
there are 60 k accounts, and a trading volume that was at 3M USD/day at its peak.
i'd be shocked if there were only 8% of existing Bitcoin on deposit at MtGox.
carry on...