Not sure I follow your question and I'm no chan expert, but I'm guessing this either indicates "b" and Q were one and the same or simply that the Q account has the ability to remove all posts in his forum.

Honestly, since the tripcodes are the same, logic would dictate Q would be able to remove the "b" post - the Q account being the same as it was during Q's last post.

Basically, I don't see the removal of the "b" post as anything other than a curiosity. It doesn't prove a thing IF the Q account is hacked.

It's a cool curiosity though. 😁

There is material information and immaterial information here.

Immaterial Information: Who is/was "B"? This isn't really important, not in the current context.

That is, the following questions are OK, but not really relevant at the moment:

If it was a hacker/interloper, well, OK. Someone managed to get in. But, then why did they not do more? Why post that ONE drop? If it was a chacker/interloper, why post under the name "B"? etc.

If it was Q, then why did they post as "B"? Why post without a trip code?

As praying medic points out, there is not enough information (at that time) to identify confidently whether this is Q or not.

Material Information: the post was removed. The board owner (Q) has full and complete authority to post and delete any content on the board. Removal of the post neither confirms or denies the identity of "B".

However, we might have conjecture about a few things:

If the B post was Q, why? Possibly, in order to be able to verify in the future Q's return (now) by deleting the B post.

Main point is, the fact that it has been removed indicates that the Owner of the Board (Q) is in fact active, and responsive to anons on /Qresearch.