It’s mentioned briefly but they seem to imply that Zeta Halo had disappeared along with the Infinity. That’s quite a big deal because it would explain why there would be a delay in an outside force intervening. Neither the UNSC or the Prometheans would be aware of the rings location. Plus I guess it also means the multiplayer season 1 is taking place six months before the campaign.
Also random and unexplained slip space jumps is a lot like the Spirit of Fire randomly being jumped to the Ark. Perhaps this Harbinger has something to do with it? They never explained what caused that in Halo Wars 2.