https://www.gamespot.com/articles/halo-infinite-is-so-ambitious-it-needed-a-new-game/1100-6459811/

Microsoft started its E3 2018 briefing with a trailer for Halo: Infinite. As we later learned, this was really more of a trailer for the new Slipspace engine that powers the game, not the game itself.

Now, 343 studio boss Chris Lee has spoken up to say Halo: Infinite is so ambitious that it needed a new game engine. Responding to a tweet from a concerned fan, Lee said, "Halo: Infinite is an ambitious new game and we needed to build a new engine to support that."