That is why we do differential diagnosis.

Any ADHD diagnosis includes checking for any other mental or physical issue that could explain the symptoms. Any when anything else is ruled out AND it can be established that the person always struggled with those symptoms (ideally that they were present in childhood) will you get a diagnosis.

Furthermore we know that ADHD has a genetic link. You have a 40% chance to have it when your parent has it. While we don't know every detail about how ADHD develops, we know it is something you are born with.

I am so sick that people pretend that ADHD is some vague concept. No, it is vague for you because you are ignorant about it.

Yes, ADHD can show up very differently in people. Which is not surprising because people happen to be different. Covid-19 can have widely different symptoms between people, doesn't mean it is not real.

As an ADHD person when I interact with other with ADHD people, yes there are huge differences but there is always a shared understanding. I never have the feeling the person has a completely different thing. There is always shared understanding.