Jay is Mr Olympia and is doing what it takes to stay up there, so what's the big deal if he can't do chins? I'm sure if he worked on it specifically, he could do 12 - 15 reps in a few months, but given how long he's been training and how frequently, I doubt he's concerned about one single exercise to prove a point no one cares about.
Besides, even if he did do 15 reps, it doesn't prove that he's healthy or athletic or anything else. Mes, I see your point, but chins don't help you win the Mr Olympia. I'm sure he's done his share of chins over the years. And I know if he could do them, people would then pick on the fact that he can't sprint or run marathons or something else, lol.
Bottom line, bodybuilding at the Olympia level isn't about health or athleticism or even strength and endurance.