He can’t prove his claim to fame (”multiple intellegences”) either. Its really quite incredible how popular that stuff got without an ounce of statistical or biological support.
He didn’t need evidence - he was selling it to “educators”, who are always looking for the latest shiny psychobabble bauble. Gardner got extra fame points because his theory was so egalitarian. The are no stupid people, just people with different intelligences. It is rubbish, but very diverse. I thought he was dead already. His theory ought to be.