I'm not saying that filtration theory is false, I'm saying it seems less likely to me than the following alternative: The chemical composition of a sober persons brain causes us to have the usual experience, but taking a substance like psilocybin alters the chemical balance and therefore makes us have unusual experiences. It seems like I have to suppose more assumptions under filtration theory, and that those new assumptions are unfounded and that the consequences of accepting filtration theory leads to predictions that aren't true (vis a vis brain damage).

Which brings us to why I tried to make the point about brain damage. Since we are working with the idea that psychedelic experience is the default but that the brain is filtering out most of it, it seems to follow that strategically damaging the filter should allow psychedelic experiences to flow through. You're right, not all brain damage is the same, but enough people suffer similar brain damage that I believe we ought to have seen by now some subset of patients with certain types of trauma reliably report psychedelic experiences as part of their symptoms. If such a subset exists, then I stand corrected.