Tower thanking FedEx for his “Professionalism” was his attempt to hopefully avoid a report of the incident, and also perhaps, acknowledgment that FedEx saved the day, and averted a disaster. If the Southwest crew knew FedEx was on final, they should be having “Tea and biscuits” (as you Brits like to say) with their Chief Pilots and the FAA. I’m sure there will be no tea and biscuits for the Tower controller.
This in UK terms both a loss of separation incident in IFR LVP and an airprox and would certainly be investigated here.
As an aside do ATIS in the USA include the phrase "low visibility procedures in progress" or anything similar.
Appreciate in lots of places you don't get the frequent grim weather we do.