It seems to me that counterfactual means "against the facts" (well, no duh), and the author is saying that what we think we know ourselves (autoepistemesis?) is in fact an illusion, that Truth is really something else entirely.
Those are ways in which I have seen "counterfactual" used, but I am still struggling to fit those concepts into the original sentence I quoted. Can anyone rephrase it in a way that makes it clear what Metzinger was on about?
Thanks Kenneth. Metzinger is German, but most of what I have read seems comprehensible. I've just discovered there are published commentaries on his book as well as a precis of the book here: P S Y C H E: Symposia - Metzinger