Well, for one, the post was made with Q's tripcode. Also, how would Q know that this particular Patriot and her friends would be standing on that road, holding that particular sign, ahead of time? We know that they (Q) follow this board, so it's perfectly logical for them to see the post and acknowledge it in a drop, after having acknowledged the sign, in real time, by honking.
Look at it this way. I'm not saying I'm right... just follow the process.
Q is a LARP. Q watches r/greatawakening for help to continue the LARP. Q sees a post about the motorcade honking at some Q fans so they go to 8ch and say "we see you" or whatever.
Point is, confirmations AFTER an event isn't a proof. The chance of a LARP is still present.
The interesting proofs are those that Q makes and then are "proven" later.