I agree that operationally DL is the best of the big 3 and that their agents are the easiest and best to deal with IME.
But the reason that the Captain thanked everyone in J is because 9,999 times out of 10,000 they paid revenue or an exorbitant amount of miles for that seat.
The craziest thing about this story is that you were booked into saver J JFK-LAX
[when it (as always) wasnt available].
But again, DL reps are more likely to help than most other airlines....