First, the prognosis for recovery with ASIA A is defined only for spinal cord injuries above the conus. You are correct to apply the same prediction from traumatic spinal cord injury to people with AVMs, conus, or cauda equina lesions.

Second, the prognosis for recovery with ASIA A is not as bad it is suggested to be. I don't know your cousin but what does he know? Has he followed up on the patients for long term? Do you really mean to say that you would rather believe your cousin than other people describe?

According to this poll, 24% of the people on this site with so-called "complete" spinal cord injury recovered additional motor and sensory function 3 or more years after injury. While that recovery may not be enough, it is not nothing.

Christopher Reeve is a documented "complete" (ASIA A) spinal cord injury and he recovered sensation in over 75% of his body and he is able to move some muscles on all four limbs. He went from an ASIA A to an ASIA C. I don't think that it is as rare as it is suggested to be.