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.
His theory ought to be, definitely, but they are still adoring it. Because of course you cannot say that some kids are SMARTER than others, it makes the DUMB ones feel bad. Or jealous. And everybody should be as smart as everyone else — if that has to be an IQ of 80, then lets make those smart ones DUMBER.