Why steal a plane if you could just buy it (assuming state level actors can gain control over a shell company with an air operators' certificate; aircraft cost really isn't an issue when for scrap value you can get old 747s with plenty of range and capacity) or charter it (leaving you with only pilots to dispose of, probably on the ground, if you're looking to do something nefarious)?

The first rule of being an armchair intelligence analyst is to rule out the ridiculously silly, and "maybe they just wanted a 777-200" falls into that category along with "maybe those 9-11 planes were just a cover for a controlled demolition"