I'm not so sure. Pullum says "But even if you ignore all the stupid stuff, the last two sentences really are genuinely ungrammatical for perfectly clear reasons." He's referring to sentences 7 and 8, which he says are ungrammatical because of punctuation. It seems to me that for Pullum, punctuation is grammar.
I dunno. He says:
First, let's look at the seven sentences of the letter above in the light of the usual kind of judgmental prescriptivism that the members of QES always purported to care about (and keep in mind here that in some cases I am applying what prescriptive authorities generally say, not endorsing it):
and then goes on to list the reasons why orthodox prescriptivists would condemn them. Only then does he say that the last two are "genuinely ungrammatical for perfectly clear reasons" but he doesn't say what the perfectly clear reasons are. I don't see what they are but I don't see any reason to believe that they are the reasons of punctuation that he had already outlined.
