What he's suggesting is that the intelligence gradient acts as the principal selection pressure in determining group membership. It does not. This particular group is self-selecting, and selection pressures come from deliberate volitional acts by its members, not from any "tendency" toward intelligence. In short, they're liberal because that's who liberals hire.
Academia is so inbred, it borders on nepotism. Making your conservative politics known prior to tenure and full professorship is the kiss of death on your academic career.