There is strong in-game evidence that Sudra is a real place, and that the in-game events are real world events and not a dream-world.
When you go to Ukkin-Na, Trace goes through a series of progressively worse hallucinations. None of these things can be seen by Elsenova, despite the fact that she can see Trace at all times. This means that the Rusalki are not inside Trace's head, and Veruska does not have him in a dream-world. If that were the case, the Rusalki could see everything that Trace sees and experiences, including these hallucinations and the Vision boss battle.
Instead, Elsenova only knows where Trace is and sends the drones to get him when he passes out. Therefore, Sudra must be real.