Illusionism, in the philosophy of mind, is the position that phenomenal consciousness doesn't exist, it only seems to.

Starting with some definitions, phenomenal consciousness is the subjective aspect of experience (for example, experience of seeing the red colour).

I'm reading a paper by Graziano: We Are Machines That Claim to Be Conscious

In the light of this paper, illusionism explains illusion of phenomenal consciousness as the introspection misrepesentation of the complex pattern of brain activity (physical phenomena). Basically, brain doesn't know how it works and it mistakes complex physical processes with phenomenal properties of consciousness.

To me, this is a compelling position.

I do have some questions, though. If there is a red apple and redness is experienced, what exactly do illusionists argue doesn't exist about this experience?

Since they deny phenomenal consciousness exists, do they claim that redness doesn't exist all (brain is somehow fooled in thinking redness exists)?

Or do they claim that redness does exist, but that it isn't exclusively subjective phenomenon? This exclusivity is an illusion. Experience is in fact objective and redness can be experienced from the third person (at least in principle). It isn't in essence subjective phenomenon.

We can connect this to Daniel Dennett's response to the knowledge argument. If Mary did truly know everything there is to know about how people experience colours (physics and biology around it), she would already know what it is to experience them. Therefore, she would learn nothing new after leaving the room. Her experience of colours came from objective route.

This is not intuitive as no real human being does have such a knowledge nor do we know absolutely everything about the brain and how does it experience colours.

It's kind of like explaining what triangle is to someone who's never seen one. The person would already know what it is if it got all the relevant information even if he/she hasn't seen it.