They didn't really get into brute force but they simply hacked into ISIS files and found "BITCOIN KEYS" to which allow them to access their bitcoin funds. that just make sense. Because i can't believe it at first if someone actually hacked and took the btcs without the key.
The big question is, if Anonymous has stolen their bitcoin keys, what are they going to do with them? Are we going to see a stash of coins suddenly appearing on the exchanges (either now or perhaps in the future)?
That is my concern as well. Will the Anonymous give the coins to authority? But which authority?