This is what John Lorber, the British neurologist that conducted the studies and examinations written about by Lewin, said about this specific case:
"I can't say whether the mathematics student has a brain weighing 50 grams or 150 grams, but it is clear that it is nowhere near the normal 1.5 kilograms."
I read that, but it's still unclear to me exactly what Lorber means by "brain." I see the term used for both the entire brain--brainstem, midbrain, and cortex--and sometimes just for the last. When I read B-Chan's post about "someone who was fully conscious despite having no brain tissue," I thought it meant no brainstem or midbrain either.
Also, from what I've read, it probably makes a difference whether someone is born with so little brain matter, or if the hydrocephalus squeezes their brain into a small space as they get older. In the latter case, it's less surprising that what's left can take over the functions of what gets destroyed.
In any case, it doesn't really say much about whether, evolutionarily speaking, consciousness was an emergent property of the development of the (healthy) human brain.