Well, I believe that leaves two possible destinations. If you are talking about JFK, the SEA-JFK fares do in fact require non-stop routing
PUBLISHED RTG SEA-NYC/DL501 /TAR-DRG1 EF-05JUN18 DIS-INDEF
TRAVEL MUST BE NONSTOP OR DIRECT
If, however, you were flying to BOS, then yes, SLC is a valid routing for those fares
PUBLISHED RTG SEA-BOS/DL139 /TAR-DRG1 EF-18SEP18 DIS-INDEF
MAP CONSTRUCTED LEFT TO RIGHT AND RIGHT TO LEFT
1. SEA-SLC/SEA/RDU/NYC/MSP/MEM/DTT/CVG/BOS/ATL-BOS
Those routing rules are directly from EF.
I got shot down for an SDC from DTW-LAX non-stop to DTW-SLC-LAX a few months ago for precisely this reason (my original booking class was available on the DTW-SLC-LAX flights, so that was not an issue). I knew the DTW-LAX fares require non-stop routing, so I didn't fight over it.
By the way, it's not always obvious when a broken fare is being presented and you sometimes have to drill down into the fare rules. For example, DL can show SEA-MSP-JFK as First(A), but if you go into the fare rules you will see it is actually a broken fare involving two A fares (one for SEA-MSP and one for MSP-JFK).
Where did you find the fare rules from EF? I have not been able to locate those. Thanks!