Look at the fare code and flight listing on the non-stops. They are clearly F, not D1 routes based on fare codes. The headings don't mean anything. They only list both on actual D1 routes (JFK-LAX/SFO, DCA-LAX, BOS-SFO). It's typical DL IT nonsensical behavior. Fare code is the ultimate decider here --
https://pro.delta.com/content/agency...structure.html
It's a J fare class. Plus in past JFK-SEA didn't show as D1. JFK-SEA showed as FC and JFK-LAX/SFO-SEA showed as D1.