Technically, any plane the president is on is "AF1". The question here is: If the DE airport is too small to accomodate a real airplane, then why didn't they just take him there in Marine 1 ? That's the question.

I don't believe that's true.. here is why, here is what I learned:
1) If the President is aboard a civilian aircraft, it is properly referred to as "Executive One". This is AF2 (if it isn't "Castle Rock AF2". 2) AF1's heavy shielding is tough enough to protect the wiring and crucial electronics from the electromagnetic pulse associated with a nuclear blast. 3) It has advanced avionics and defenses and they are classified. 4) AF1 is outfitted with electronic counter measures (ECM) to jam enemy radar. The plane can also eject flares to throw heat-seeking missiles off course. Etc..