And if his IQ had been scientifically tested and the number was low and public, they would have written it off as #FakeNews or just not cared.

If all the candidates started volunteering up their IQs and it became a political norm, they would rig tests to give them high numbers or higher sketchy scientists to test them (just like Trump did with his health). More importantly, the number would receive way too much focus. IQ isn't a good way to measure people for really any kind of job. Past performance is always more useful as an indicator of future potential, not test scores (which is what IQ is, you may think it's a very good test, but still just a test score). Most politicians are lawyers, should we make it a political norm for them to reveal their LSAT scores or their law school GPA? That information is basically useless to voters because the type of intelligence measured by each number is only one of many things that make good, democratic leaders.