Exactly—a serving General should NEVER answer yes to a question about his political ambitions. Whatever his real ambitions, Petraeus did the smart thing here (of course).
It would have been appropriate to ask Weasely Clark the question during the time he was a Clintonista acolyte within the US Army and NATO.... but of course, IIRC, the MSM never did ask such questions of Clark, even though he owed his last promotions and prominent role to the fact that he was/is such as weasel for the Clintonistas.