It's somewhat fair, yes it's only one game against a rather weak offense, BUT you've gotta give Williams some credit here. His resume speaks for itself as far as being a defensive coordinator goes. And who knows, far worse have had second shots as head coaches (Norv anyone?).
If anyone remembers Bill Belichick wasn't very successful his first go round as a HC (36-44 overall). Williams is a bright guy, I can't see how he won't get another shot at being a HC someday. Hopefully it's here after Gibbs is done with his second stint and added a few more SB trophies to the case.