https://www.gamespot.com/articles/e3-2018-halo-infinite-trailer-was-only-an-engine-d/1100-6459632/

Microsoft started its E3 2018 briefing today with a big Halo reveal: Halo Infinite. We saw a massive, sprawling world with creatures we've never seen before in Halo. The video ended with a tease of Master Chief. But as it turns out, this was only an engine demo, not necessarily indicative of what the game will actually be. The engine is called Slipspace, and what was shown in the trailer is the culmination of "years of work," developer 343 Industries said in a blog post today.

343 created a new engine Halo Infinite because the game was so ambitious that the studio new technology to realise the game's ambition. "The E3 demo showcases some of the exciting potential of this technology--everything you see is running in-engine," 343 said.