Pre-COVID, the 75S (with flat beds) often appeared on JFK-SEA and JFK-SLC marketed as first class. Marketing domestic flights as first or D1 is almost academic right now given the lack of inflight service either way -- not sure if anything more has come back recently but when I flew JFK-SFO D1 in December, the only thing that made it D1 besides the seat was the upgraded blanket and pillow. No amenity kit, same food and drink as any other domestic F flight, and the D1 seats always come with the "upgraded" headphones regardless of marketing since the seat needs a dual prong input.
It looks like right now the 339 is doing mostly Asia routes out of SEA plus JFK-TLV, so the JFK-SEA leg connects the two to move the fleet around. Then in terms of Delta's decision to label it F or D1, they seem to like to charge pretty steep prices for domestic D1 vs domestic F (regardless of whether any D1 services are actually being offered), and with a likely leisure-heavy customer base this summer plus a much larger than usual number of premium seats to fill, they probably figure they won't be able to sell enough seats at D1 pricing and would end up having to give it away for free as upgrades.