I would agree that the sack numbers are deceiving. PFF has him ranked 89th out of 121 qualifying DT's on pass rush win rate (winning against a blocker), and 92nd out of 121 on the same stat against true pass sets (essentially drop back passes). One benefit he has had is that he has seen the 10th most drop back pass attempts among DT's, providing more opportunities to get after the QB when they are likely to be holding the ball longer.
He's only in his second year, so I wouldn't say he is a bust or anything close to that, but I agree with you that the sack numbers lack context.