Screenshots? I'm guessing OP select "Delta One" in the advanced search form, but it displayed only F flights because that's all that's available. Sorry, I don't see the case here.
OP posted there was one D1 routing provided, via JFK. Not sure how many people would wast time flying to JFK, to goto SEA, but that's DL. OP doesn't say which flavor of Delta used to book. Some lump First/D1 into one column, although most have gone to separate columns. Thus you have to pay attention to if it say First or D1 as the cabin name.