If only Q can post on that board, then logic would dictate that it has to be Q.
BUT… if it were Q, why would he not use his tripcode and Q?
So which is more difficult for a person to do?
Gain "permission to post" for an 8kun board, or know Q's tripcode?
I would assume that knowing Q's trip is harder.
Thus I think this is not a legitimate post.
Then again I'm only thinking out loud here and trying to think of it in terms of sheer technical logic, and not on personal motives and psychology – maybe it's some kind of fake out from the white hats?
Again, just spitballing here