This looks like a good example of the 80/20 rule: 80% of our brain matter is dedicated to the hardest last 20% of IQ points. (Only being slightly tongue-in-cheek here.)

Also, there's probably usually a lot of brain matter dedicated to redundancy, memories, and enabling connections between ideas to foster creativity, and this person may be lacking in these areas in hard-to-measure ways. If most of our brain truly had no purpose, evolution would have selected against it forever ago. (Instead, evolution went through large lengths to keep our heads so big.)

I agree this could be the other side of the coin. I'm not totally one sided on this. Its possible that consciousness and sentience arises from sheer complexity, I'm just not totally convinced, and there appear to be outliers and unaccounted for anomalies. In this case, 10% = IQ of 80 and totally functional within society. Pretty astonishing still.

This argument works just as well (or badly) against the brain is just an antenna hypothesis..

I agree. I point this out purely as an outlier that doesn't fit well with the "you are a product of the complexity of your brain matter, period." In this case, only 10% of your brain! Pretty astonishing. The man was totally functional in society, even though he had an IQ in the 80s. A broken antenna still gets most of the signal maybe, just not the high definition stuff.