Everything anyone says right now is fantasy, but it's fun to come up with ideas before we get the official word from 343. It's great to see people tossing ideas at the wall.
Atriox has no slip-space capable ships with him at the Ark according to canon. The
Enduring Conviction was bisected by drones, but Atriox also had several Corvette class ships at the Ark which can enter slip-space. The problem is the portal they used to get to the Ark is disabled and their own ships don't have the energy to generate a slip-space bubble that would carry them all the way back to the Milky Way galaxy.
They at the end of Halo Wars 2 are indeed stranded, but the Ark can bring ships to it through slip-space which is exactly what it did to the Spirit of Fire. I assume Atriox also used one of the slip-space bubbles generated by Forerunner technology to reach the Ark as no Covenant or UNSC Vessel, even a super carrier like the
Long Night of Solace was only capable of inter-galaxy slip-space travel. The only vessel the Covenant has possession of that could make the trip to the Ark under it's own power was the Forerunner Dreadnaught (Keyship) that powered
High Charity.
If it can do that there's no reason to assume either Cutter's crew or the Banished can't gain control of that mechanism and escape.
What you said does make sense though in context of the message in the teaser. Perhaps the Banished assume Atriox was lost when the slip-space bubble leading to the Ark was disabled and that's why the voice on the recording refers to their actions as executing his will.
Well I'm even more excited now. Thanks