[icambron]

I think that sort of misses the point of the metaphor as used here. If one big part of the brain is missing, and thus the mechanisms it enables are unavailable, some other part of the brain (at least in this case) makes up for it by providing an approximation of that mechanism. That other part of the brain is thus not able to perform whatever its normal function would be; even if it's doing double-duty, you expect that there's some capacity limit. When and if that deprioritized function involves reasoning, then you'd expect a drop in reasoning ability. Thus the "processing power" metaphor is simply saying that if a neuron is busy doing something, it can't also be busy doing other things.