Yeah, I noticed that. Perhaps the Enduring Conviction was the only ship deployed to the Ark?
Where was the Banished Fleet in Halo Wars 2? It seems like the Enduring Conviction was the only ship that the Banished used in the game. There's no way a faction as strong as the Banished could only use one ship.During the cutscene when Let 'Volir is kneeling in front of Atriox, there are several battlecruisers surrounding his flagship.
Although where they are now is anyones guess.
Let 'Volir running abilities in the game seem to center around teleportation and stealth, so I don't think it would be too outside the realm of possibility for him to have some stealth ships in the area. They exist in lore, though you don't see many from the covenant perspective.
We also don't know what the Human presence was on the arc yet. It would be silly to think that after what went down in Halo 3 and Hunters in the Dark that the UNSC would not have small fleet stationed there. Maybe there was a battle with mass casualties on both sides. Who knows :)
However, for story sake, he cant have much if any of his fleet in the area when Spirit of Fire is the only human ship in the story right now. The fact that the Assault Carrier didn't snipe it with an Energy projector is already enough plot armor to be distracting.