Totallyv agree. I saw his posts and knew he read the books or spoilers.You say that, but linking Olenna AND the necklace to the poisoning is damn near impossible if you have no foresight. That's one hell of a lucky guess to figure out before Sansa even notices the missing gem.
He also manages to mention not seeing Littlefinger for a while, mention Sansa is fleeing and mention how much he cared for Catelyn and asking Sansa to flee with him before he left in the same post.
There is just no way you get that lucky on your own without knowing what happens.