I'm a psychiatrist.
He's not entirely wrong (although he's right for the wrong reasons)
Junk food, video games, screen time, and any other high reward/instant gratification type activity causes a huge dump of dopamine into the brain.
Over time the brain will start to down regulate and remove its dopamine receptors so it's not constantly over stimulated.
A lack of dopamine or the ability to detect it is essentially the root cause of ADHD. Except ADHD itself is probably neuro developmental rather than acquired.
It is perfectly possible to give yourself an ADHD type syndrome by doing these things for long periods, but we wouldn't necessarily think of it as ADHD as such.
It would very likely be reversible if you stopped doing all the high stimulus things, but extreme exposure during childhood may well mean the brain is irreversibly changed from what it would otherwise have looked like.
And as someone else said, these things would definitely make pre-existing ADHD worse
(Yes, I realise this is an incredibly over simplified version!)