Is he? Or is he benefiting from ridiculous goaltending when he's on the ice? Goalies are sporting a .950 save percentage when he's on the ice at 5on5. That's leaps and bounds higher than at any point in his career, thus accounting for the goals against when he's on the ice.
On one hand, his "against" metrics are some of the best of his career. On the other hand, that doesn't say much considering he's still way down compared to other centers in the league. For instance, he's ranked 57th in CA/60, 62nd in FA/60, 61st in SA/60, 84th in SCA/60, and 60th in HDCA/60 among centers who've played at least 300 minutes at 5on5 this year.
So I'm not really sure I buy the whole "he's not producing like he should, but at least he's been great defensively". It's closer to "he's not producing like he should, but he's gone from playing mediocre defense to middle of the pack defense".