My thoughts on the whole ordeal are:
1) There are graphics rendering techniques going on here, like ambient occlusion on the weapon models, that were not present in previous Halo engines. The textures and some models/geometry look slightly placeholder (which is common for a game at this development stage) but the hud and weapon models look polished as hell.
2) People post crappy fake "leaked" screenshots and videos that are photoshopped and modified every day, and bungie and the ninjas NEVER paid any attention to them. Now, suddenly some VERY legitimate stuff comes through the pipeline and they start dropping the banhammer like thor. Obviously, that speaks volumes.
3) The incredible amount of press coverage the leak got. It was on every popular video gaming blog on the 'net.