Apologies for dredging up an old thread, but I am having an issue with the same fare rules language contained in the OP.

I am trying to go TUL to CPH this summer and there is a low fare shown on ITA Matrix of TUL-LGA / JFK-CPH. However, Delta website and other OTAs will not price this itinerary. TUL-NYC-CPH is shown in the routing rules and the fare rules contain identical language quote above. Ultimately, I assume if the website won't price the trip, it's a no-go, but I'm just curious why that might be. It won't let me do a multi-city trip either, it just errors out.

Thanks for any insights!