Cory likely doesn't have high expectations so that they can brag about the extra teraflops of the Series X. And doesn't have an interest in trying to downplaying Xbox Studios or what the Xbox SX can do. He's a gamer who happens to also work for a studio owned by Sony. He probably just sees it as another Halo game that looks better than their last and doesn't expect every developer to have amazing breakthroughs in graphics graphics/art like his studio does.
I had low expectations coming in. All I expected was that it was a improvement from Halo 5 and it was and that the Series X version would be more stable, better picture quality, more detailed, and eventually get ray-tracing. I liked the demo visually simply because it was an improvement over the last Halo game. I don't expect the company that can barely stabilize MCC is going to suddenly become one of the best teams at producing visually stunning games. Especially if they keep mostly the same employees. Employees get better over time and adding new employees help but if the bulk of your team already isn't really the top tier of the industry, they will hold each other back. Which is why I'm not a huge fan of Microsoft's studio strategy of acquire, fund, and laissez faire approach of B-grade studios. Because you have to keep all the crappy employees to work with your new better ones and the same management/leadership. You'll probably see improvement but you don't fix/remove any of the problems working at the existing studios. Not saying they should go in and fire people because that ruins morale but there's a benefit in forming brand new teams from scratch over trying to improve an existing team that didn't have a lot of potential to begin with. The turnover at 343 from a few months ago is hopefully the result of something positive for the company.