I have to agree with you on the diagnostic part The study may be valid, but letting someone with a brain injury sleep before arriving at the hospital is something I would not do.
The last time I dealt with an individual who had struck their head many miles from the nearest hospital, I kept asking questions to ascertain their mental state on the 45 minute hike out. I kept the routine up during the hour long drive to the hospital.
It was a good gauge of their mental function, and gave me room to change plans if needed. I also wound up with a good set of responses to hand over to the medical staff for diagnoses when we arrived. e.g.: They are lucid now but 20 minutes ago they could not tell what their birthday was and an hour before that could not remember who I was.