Apparently Covanent Canon pointed out on Twitter that the visuals in the Official Gameplay Trailer look somewhat better than the Campainge Demo.
I also jumped on the gun thinking this was the final build, but came to realize that this is most definitely an old build they had made for E3 or even prior to be shown. People have already pointed out the difference in screenshots compared to the demo which basically confirms that it was prepared beforehand.
They are also working at home which makes it harder to develop this game and probs decided it was better to continue to work on the game rather than construct a completely new build.
Although they should have made it clear that it was an old build if true, but i have faith that the current build that they have looks a lot better. it also seems that they worked on AI before the textures as i have heard they can do pretty advance stuff already like throwing grunts at you and picking up different weapons to kill.
Just have faith and dont automatically assume this is the final product, i highly doubt 343 would risk -Yoinking!- this up.