For a purely semantic sake, you’re probably right. But for a colloquial sake, the term “valid” here, doesn’t mean “legally valid” or “medically valid”, but instead means “emotionally valid.” For some people, confirmation is therapeutic enough to help. Also “diagnosis” doesn’t exclusively mean “medical diagnosis”. There are many definitions to the word, and in a medical sense, it usually means what you’re describing. But “I think I have ADHD” is a diagnosis. Not a medically valid one, but something that might help me get through the day sometimes. And if that’s all I need, then it’s emotionally valid.

Being told “your self diagnosis is not valid” to some people is the same as being told “There’s nothing wrong with you.” (Because most people aren’t working on a strict legal medical definition of “diagnosis”) Emotionally validating your assessment that something is wrong can very well be what drives people to advocate for a medically valid diagnosis.

Also, saying “You don’t have ADHD unless it’s diagnosed ADHD” is wrong regardless of stance on self diagnosis. If my arm is broken, it is in fact broken, even if it hasn’t been diagnosed. Undiagnosed issues are still issues. Too many anti-self diagnosis claims come across as saying that if you don’t have a diagnosis it doesn’t exist. At most you can claim “You don’t know for sure you have ADHD unless it’s medically diagnosed”

As with all things, a self evaluation is a useful “what do I do next” step.