Expecting this to be >6 hours for the main campaign seems a bit optimistic to me. I'm expecting 4-5 hours, with even 5 hours pushing it. Ground Zeros was originally in TPP as a prologue. Honestly, how long are MG prologues usually are? They sure as hell weren't 6+ hours affairs. This is a 4-5 hour demo given premium pricing.
Originally it was always a separate game.