He's not positing it as a given. Hence the term 'may'. And his whole point is that brilliance alone confers no guarantees of common sense or competence.
I had a partner at one time who was 'brilliant' by most people's definition - board certified in internal medicine and rheumatology. One Monday (early '90's) she let us know how proud she was of the vehicle purchase she had made over the weekend.
She told us how she'd spent considerable time studying safety ratings of family vans as she had small kids & wanted the most highly rated one. She was also frugal, so she wanted to purchase a used one in good condition. After completing her research, she had decided on a Ford Windstar, so went to a local Ford dealer & asked to see his inventory of used vans. She settled on one & bought it.
She wanted to show it off when we left the office for lunch that day so we accompanied her to do the courteous thing & Ooh/Ahh over her new/used Ford Windstar. Only one problem: it was a Chevy Aerostar.
When we gently informed her of that fact, a shocked look of disbelief crossed her face and she said (I kid you not), "Wait a minute. I bought it at a Ford Dealer. How could he sell me a Chevy?"
The topper: She actually took it back to the dealer that evening & got a Windstar. Not sure what explanation she gave, but I'd love to have been there to hear it.