Many of these hashes, even those that appear to be strong passwords, show up on various websites about password brute-forcing. Notably, 2 days ago, a user named georgeclooney posted requests to crack some of these hashes on the InsidePro password recovery forums. He is almost certainly the same person who attacked MtGox.
Emphasis yours. Can you elaborate on why you think georgeclooney is the attacker? Is that based solely on the fact that he posted some of the hashes before they were publicly leaked?
Yes, it is based on this fact.