I can see how that would be a problem, but it's a slightly different scenario from the one posed by the OP.
Most of the CVs I see show publications that range from portions of the dissertations to related work. Often, graduates produce essay-length work in research seminars, etc., or chapters that don't really fit into the final diss.
That work can be published with no threat to the book contract. More, since the advice here has been to publish 1-3 essays, it sounds as though the person who published 6 (all from his dissertation!) really went overboard.
So I take your point, but I don't think it contradicts the general advice here at all.
No, I don't think it contradicts the general advice but I do think that it is worth mentioning to the hyper-types that you can publish yourself into a corner. Particularly for those in fields where the "outlets" in terms of secondary or tertiary level journals abound. A frantic summer of shaping every chapter into a separate article (which is what happened here) might not be in your interests simply to pump up a publications count.