If consciousness is physical, then we are radically mistaken (suffering an illusion or cognitive error) if we think that our present conscious experience conceivably exists independently of a physical substrate. But it seems like consciousness is the sort of thing that could exist without necessarily depending on a physical substrate. By conceivable, I just mean that you can coherently conceptualize consciousness without a physical body or brain, and so the two concepts (the physical and the mental) can be decoupled without losing any resolution to the concept of either. If consciousness is the sort of thing that exists as a property of physical systems, then we are radically mistaken in thinking that consciousness can be conceptualized apart from the physical structures that are thought to instantiate the phenomenon in question. I also take it that brute a posteriori identities (of the sort articulated by Type-B physicalists) are incoherent. So consciousness is not something that necessitates the existence of a physical substrate (at least, not as an a-priori metaphysical necessity)

I basically don't think that this is a problem because by definition phenomenality only refers to what you are directly aware of / experiencing. Deeper facts about the underlying physical structure of my brain are not going to matter. And I don't just have a vague sense that my consciousness could be something outside of my brain. My position is that the certainty that I have about my own consciousness is way higher than my certainty about the existence of anything physical existing at all. This is known as Hempel's Dilemma. Descartes says "I think, therefore I am", basically saying that his own mind was the only thing that he could not doubt. If the existence of the mind is not absolutely certain, than it is about as close to absolute certainty as I can possibly conceive of. And I must attest that what I am is not just a mereological aggregate of physical parts and their interrelated activities, but a unified, immaterial self. I think that qualia are undeniable, that experience is the baseline for morality and the ground of all knowledge. That the denial of first-person consciousness is the most absurd thing one could possibly believe (indeed, it is impossible to believe, because belief requires phenomenal experience of some sort). Also, I take modal rationalism to be the most plausible view of modality. I don't think that possibility can be distinguished from logical possibility in any coherent way.