Great question. My simplest answer is that he was able to outsource his executive functions to others. He started out in daddy's business, with millions of dollars showered on him (much of which he lost on bad deals). He was able to hire expensive and unscrupulous lawyers who wrote contracts for him which often resulted in everyone else being stiffed. Despite claiming to be the master of the deal, he wasn't the one handling the fine print. He's even acknowledged to biographers and others that he's never read a book. Even as a parent he boasts of how he outsourced all of the hard and detailed work to others - never changed a diaper.

And his ADHD has been an asset as a politician. He knew how to promote his propensity for speaking off the top of his head, as showing that he was honest, unlike other politicians, who had to think of what they were saying, and censor themselves. But this was ADHD-candor, and not really honesty - as shown by what an outlier he is in out and out lying.

I've known doctors, lawyer, and architects with ADHD. They usually succeeded in their careers despite their ADHD due to some combination of smarts, good support from others, luck, and charm. But often their ADHD meant that they ran into troubles that their peers didn't - being late for appointments, careless mistakes, speaking out inappropriately at meetings.