What I do know, is that it wasn't someone from this forum.
How can you know that ?
Because I had my bets that they'd be celebrating about it on this thread by now if that were the case. Pretty safe educated guess to make if you ask me.
I would agree. If they are a regular in the discussion of this challenge, they'd at least let peeps know the priv key once all forks have been taken care of, especially if they follow on here and know people like to know the priv keys to try and "create" a pattern with the challenges.
Also, if it's somebody not on this forum, it seems like they would at least come forward (on some venue) as this is a new world record. Maybe sooner or later they will. Time will tell...