I don't believe in astrology; I'm a Sagittarius and we're skeptical.

The line is a self-canceling joke: Clarke professes disbelief in astrology while simultaneously invoking his zodiac sign as an explanation for that disbelief. The humor comes from using astrological reasoning to deny astrology, exposing how easily people can slip into the very frameworks they claim to reject. At the same time, it fits Clarke’s public persona as a scientifically minded skeptic who enjoyed witty, aphoristic put-downs of pseudoscience. The quip also plays on the stereotype that Sagittarians are blunt or independent-minded, turning a horoscope trope into a satirical comment on credulity and confirmation bias.