Not really. Hydrocephalus doesn't actually erode the brain — the cerebrospinal fluid is not corrosive (quite the opposite). If it occurs slowly enough, the brain adapts (as it is wont to do), and there is quite a bit of redundancy in the brain. The erroneous premise proposed by the paper assumes that this margin of error is grossly violated by this "90% missing" figure — we don't really know what the lower threshold for consciousness is. People have gotten by with half of their brain missing (hemispherectomies), and if it occurs early enough (early childhood), they live normal lives.
Note also that while the civil servant is not impaired, he is also not a genius (IQ 75). This indicates that while he is normal, he may be operating at or near the limit of his redundancy.