Agreed! The difficulty with consciousness is that there is no observable effect to distinguish between, say, actual pain and simulation of pain (acting like you are in pain).

And I don't think I have a good handle (much less a coherent definition) on what it means for consciousness to be an illusion. What I think it means is that the process that is getting signals about the environment, and making decisions about what to do, is getting a signal that it is in pain. The signal causes the process to alter its behavior, and one of its behaviors is that when it introspects, it notices that it is in pain. The introspection (how am I feeling) is just a data processing loop, but that process, which is responsible for tracking how its feeling, is in the pain state.

There's a lot of hand waving here, which is why this is the Hard Problem of Consciousness and why this paper has not solved it.