According to the Icelandair cargo schedule, 794 is a cargo flight.
I didn’t see SDF listed in the schedule but I did see SWF and a destination with a code of DOL. I don’t know what airport DOL is.
I was wondering the same thing when I saw Icelandair in Miami once. it turned out Icelandair regularly charters out its airplanes. In Miami’s case they were flying for the Venezuelan airline Santa Barbara