The more I think about it, I think I have a theory as to why it looks so average... See yes it's being held back by xbox one but we have red dead redemption 2 that runs on the xbox one and that game's environments, lighting and etc still looks a lot better than Halo Infinite. My theory as to why I think Halo looks the way it does is because they are targeting 60fps on the Xbox One as well like they did with Halo 5. And because of that, they had to sacrifice in visual quality since a lot of the open world games this gen and probably next gen as well are running at 30 fps and sometimes lower.
How does that explain it? The footage we saw is for the xbox series x, who knows how much worse the xbox one version will look.