I’ve not seen this overtly specified anywhere, but paying close attention to your surroundings while in the Lair makes it pretty clear what’s happening. Argos is being eaten by the Leviathan.
The Vex we fight in the lair are not on the Leviathan, they are in the mouth; meaning the part that eats worlds. You can tell after you’re shot out of the canon and you land on the rock: In one direction is the mouth of the leviathan and then outer-space beyond it. In the other direction is just a huge fiery area, this is where worlds get “eaten” and transformed into Calus’ purple wine stuff that is all over his throne room.
Argos is a planetary core, which likely means it is the Vex mind in charge of actually machino-forming Nessus. With this in mind, and considering the surroundings described above, it seems to me that the Argos battle arena is actually a chunk of Nessus which is moving through the Leviathan “mouth” to the “digestive system”. This is why the lair is called Eater of Worlds; because that is literally what is happening, the Leviathan has begun eating Nessus and scooped up Argos in the process.
Now, why we feel like we need to go kill a bunch of vex which are about to be turned into sticky Cabal wine is a different question…