Gwern was skeptical, & noted that an IQ of 75 [1] in this case study is very low. He additionally raises a few points, including that volume loss is not the same as neuron loss. He also predicts several deficits the case studies didn't report that he'd expect to see, including many small deficits in simple tasks adding up to large deficits in complex tasks.

[1] https://gwern.net/hydrocephalus#sn3