The original paper did not say that a huge percentage of their brain was missing [1], that was the journalist's flourish based on their own misunderstanding.

Tissue can be compressed, stretched, reorganized, or displaced especially to compensate for a congenital condition - the patient's brain had a lifetime to adapt to hydrocephalus, which pushed on the other brain tissue. The gray cortical shell is clearly visible in those images and their volume on a scan is not representative of neuron count or synaptic capacity.

There are far more dramatic cases of brain damage and neuroplasticity that reorganizes major functions, but there are a lot of caveats.

[1] https://www.thelancet.com/journals/lancet/article/PIIS0140-6...