They attempted to make their recent engine deterministic, and failed miserably - at least on PC: https://www.reddit.com/r/halo/comments/r78moa/halo_infinite_...

Attempted deterministic engine, no desync detection or reconciliation, and support for the whole PC ecosystem - I couldn't believe it when I realized what they were doing... pure madness (not in the good way).

I suspect it actually works fairly well for players on console, who aren't playing cross-platform (since they're working with basically the same hardware everywhere).