I alluded to the arming. As to not being fueled, not buying it. It’s standard procedure for military planes to be topped off after flight so that they’re ready for whatever mission they might be called upon for, and it’s standard procedure for all planes to be kept fully fueled in order to minimize moisture condensation in the fuel tanks.
I’ve read that range figure for the F-15E, but I have to wonder about it when the standard air-superiority F-15s list a combat radius of 1,200 miles, and the F-15E carries about 75% more internal fuel. That figure may relate to a lo-lo-lo mission profile (jets are fuel guzzlers at low altitude) or Hi-lo-lo-high. Also, you don’t have to carry a max load of ordnance. Further, they could likely have gotten there faster by going in (at least while over the Med) supersonic at altitude at minimum afterburn, and scrambling a tanker or three for the return trip.