I doubt this is the thief's wallet because it would mean that they somehow managed to execute withdrawals of 3k and 5k bitcoin 7 and 5 days ago respectively - M

I doubt this is the thief's wallet because it would mean that they somehow managed to execute withdrawals of 3k and 5k bitcoin 7 and 5 days ago respectively - MtGox has stopped all withdrawals for weeks/months now. The number of transactions seems low as well, the malleability bug would have involved thousands of smaller transactions. It would also be insanely stupid for a sophisticated hacker to funnel their loot through a single wallet. Someone who understands transaction malleability enough to exploit it likely won't be making that mistake. It isn't hard to pick out a number of bitcoin and then dive into the blockchain and find a wallet that has a same/similar number of bitcoins. Similar types of misidentification happen during the Silk Road and Sheep Marketplace heists.