The figures (150 verbal IQ, etc.) seem to be plucked out of thin air, and the central idea---that there would be an IQ 'threshold' separating people who can be

The figures (150 verbal IQ, etc.) seem to be plucked out of thin air, and the central idea---that there would be an IQ 'threshold' separating people who can be successful/good writers from those who can't---similarly backed by nothing. Where is even a hint of what a mechanism for such a threshold might be?