So in the new trailer we learn that, assuming Isabel is telling the truth and she isn’t secretly ONE OF THEM, that five months ago the portal to the Ark closed. In the artbook Halo Mythos we learn that the Ark had an emergency failsafe which activated when Cortana began her attempt to claim the Mantle. Obviously the Forerunners have some failsafes for the event of an AI takeover. So we know with certainty that the game is set five months after Halo 5.
Whats more interesting for me though is the part about how Atriox arrived one month later. We can infer a lot from this. Atriox clearly didn’t intend to go to the Ark before Cortana’s uprising otherwise he’d have arrived before the portal closed. This likely means that he has travelled to the Ark specifically in response to the Created. Given that he isn’t with any Created or Prometheans its then probably safe to assume that he is acting against the Created. If he had arrived before the portal closed he could have simply been unaware of Cortana, but clearly he must be aware if he was in the galaxy for one month post Halo 5.
It also means Atriox could not have used the portal and assuming its the only portal (and its unclear how he could have opened it without a Keyship). Which means he must have used another means of transport. The only thing I can think of are the Star Roads and possibly something new. Halo Mythos does mention enemies moving through paths unseen to Cortana, which implies something not connected to the Domain or the Forerunners. Maybe some of the Star Roads survived inside slipspace like the Dp,aom?