can not conceive you you can be mistaken about the fact you're having one
The experince of having an experince can be wrong. If the existinece of experience is the content of the experience - and you have written that the content can be wrong. If one mistaken about the very fact of experience, is that valid an experience too?
AB syndrome doesn't seem any more significant to the issue here than a synesthesia or schizophrenia. That their reported experience has no correspondence to the outside world isn't relevant to whether they're having an experience and its nature.
The experince of having an experince can be wrong. If the existinece of experience is the content of the experience - and you have written that the content can be wrong. If one mistaken about the very fact of experience, is that valid an experience too?
https://en.wikipedia.org/wiki/Anton–Babinski_syndrome