No, actually, trying to measure a skater's defensive acumen by goals against is flawed. It's every bit as bad as plus/minus because like that stat, you're discounting the most obvious thing--the goaltender.
He was one of the few good defensive players on a bad defensive team, and the puck moved in the right direction when he was on the ice. The fact that he and his linemates didn't really have the talent to score at a regular clip or really sustain drives in the attacking zone shouldn't really negate that. People think if he's some great defensive forward then he should have stemmed the tide all by his lonesome. That's not how it works, and besides, it's generally agreed he played higher in the lineup than he should have because of depth issues.
Penalty killing is also a misnomer. Bobby Holik was regarded as one of the best defensive centers of his generation and never killed penalties in NJ. And we'll see if that stands...I'm guessing Babs will try him in that role, and I think he'll be quite good at it. I don't think we necessarily have to assume Bednar didn't use Kerfoot on the PK because he didn't think he wasn't any good at it.
I really do wish though, that while I understand what they were getting at, the EvolvingWild twins didn't insist on mentioning Kerf in the same conversation as the Selke. It's made things way, way too polarizing, and besides which, I don't agree with it. He is a very good defensive player, but he's not in the upper echelon of talents in the league. I don't consider him to be some kind of elite shutdown guy, he's a good utility player who can play a defensive role and make plays at the other end. He was miscast as a 2nd line guy (or at the very least, a guy who had to drive the play on his line) but despite that, he still produced as a pretty good clip.