We nevertheless have a concept of smartness and able to judge - imperfectly and subjectively - other people on this perceived dimension.

Take, for example, Elon Musk in SpaceX. Elon knew enough of rocket science to assemble the early team and participate as a self-assigned chief designer. In many cases such actions would qualify a person as a smart one.

Yes, it's hard to objectively measure - very hard perhaps. Yet we can still allow that some of those billionaires are smart at least by some reasonable measures.

My point is that the evaluation of Musk in that particular passage seem to be also subjective, and other people might have other opinions. We don't see good evidence here - and have some kind of counter-evidence elsewhere.