Russell Barkley is a world class ADHD expert and he says those tests are not meant to be used for diagnosis of ADHD, they are just one aspect. So I think if you decided to look for a second opinion you could have this taken into account. More info here:
I find that for my DS some screen time can be regulating, especially if it is redirecting him from a situation which is getting too much for him, or providing a break/some familiarity and control for him after something which was challenging, but if he has too much then it overstimulates him, or I think what happens is that when he's so focused on the game it causes him to be less aware of his other body signals e.g. toilet need, hunger, wanting to move around etc, as well as emotions building up. It's all a bit of a trial and error process really, but with my older son, before I knew anything about ADHD and regulation etc I was really concerned that I didn't want him to be one of those teenagers who punch holes in walls because of frustration about a game, so we had a very clear hard line about violence/anger around the game and we used to stop it completely. I don't have a lot of hard lines in general but that was one and it did seem to really build a whole clear line around games are only going to work if you take a break, rather than continue to that point of uncontrollable frustration.
For my DS who is this age now, I did start removing a small amount of screen time as a consequence for violent/unsafe behaviour - I avoided doing this for ages because I felt what happens in a meltdown shouldn't be a discipline issue, but I noticed that it was only ever me, or his younger brother that he would hit. No matter how frustrated he was with DH, he would never hit him. And it's not like he's afraid - DH is generally calmer and more gentle than me! So I surmised that on some level, he felt it was acceptable to hit us and not to hit DH. And honestly, since I made that decision and when I do enforce it, it makes a big difference and I don't feel that he's more dysregulated overall, but it has much reduced the scenarios where I get overwhelmed in response back to him, which probably does help with overall regulation. And it's made things better for my youngest.
I don't threaten/warn in the moment because I don't honestly expect it to change his behaviour at that point. It makes it feel like a weapon I'm using against him, which is just likely to cause him to fight back more - I still just do de-escalation in the moment. I just made it clear in general which behaviours are considered unsafe and not acceptable and I would inform him about the consequence afterwards. But that also means that I can just remove a small amount, because I don't need him to be scared of it because I'm not trying to use it for behaviour control in the moment. It's really more of a token thing to back up what I'm saying about hitting not being OK.
I do appreciate that if you don't have other tools for him to regulate with then the prospect of taking away screen time might feel counterproductive, but I think maybe because we've always had it as a time limited thing in the first place, I don't feel that we are reliant on screen time as a tool and therefore we can have some rules and conditions attached to it and at least for our family, that seems to help it be more of a positive force overall rather than a drain. I know everyone's situation is different and you have to do what works for you and yours. x