just because a connection appears on dl.com flight search results for booking doesn't mean it's a single-component fare
I was actually able to find fare rules for SEA-JFK: ROUTING 129 FROM-TO SEA-NYC* FROM-TO SEA-RDU/BOS/SLC/MSP/MEM/DTT/CVG/ATL- NYC* (Fare Code LA07X0VP which books into I; also UAUNX0MP) ... LAX is conspicuously absent from the list of valid connecting points for this routing