Can what they say be proven? If they are brain dead, how can they remember things happening? Why couldn't they have just had a hallucination when they came back into consciousness?
I'm guessing you can't necessarily prove them, but with the consistent themes of seeing your body, the white light, peacefulness, there could be some validity to it. Individual hallucinations might be unreliable sources, but when a group of people consistently have the same hallucinations, the possibility of a spirit could be considered.
I'm guessing they could remember things through spirit and there's no definite answer to whether they just hallucinated on awakening. I haven't read into it and I'm not defending it, I just throw things out there to put a different perspective on things aside from the normal arguments.