Are you sure about braindeath being a mental condition? My thoughts, confirmed by quick googling, was a medical-legal standard for death based on permanent death of the brain. As a person can otherwise be vegetative/comatosed or whatever, or like a liver can keep operating for a while etc.

As an insult I think it's meant to be taken as "This is only explainable if you have no brain activity" not "you are like a living human with a different or damaged brain".