The training I got as a lowly ppl, was that a takeoff clearance meant that the runway was yours to do with as you pleased within reason. If the southwest crew needed to sit on the runway for a few moments getting setup they had permission to do it. Their job is to follow atc directions, which they absolutely did.

If the controller wanted you to do an immediate takeoff they normally make damn sure that you are able and you know the context. At least in my experience mixing it up with jet traffic at Boeing field.

Something like: “Southwest xxx cleared for immediate takeoff, you have a FedEx heavy on 3 mile final”

To the pilot that is saying: you can go, but you only have 90 seconds or so.