I think the larger issue is that the entire battle they described could not have happened. Chief crash-lands, gets picked up and fights the Covenant; he never gets captured by the Brutes, and there's no timeline for the days the seige took place in "Believe" for it to occur before he jet-sets off to the Ark.

Halo 4 didn't ruin its canonicity, Halo 3 pretty much obviated it (it's clear that they didn't give the marketing people the full story of Halo 3, prolly for good reason.)

Doesn't mean you can't partition it a bit in your head--maybe the Museum of Humanity does exist and those were real soldiers, they just couldn't have fought in a battle described in the diorama.