"Every X knows" is usually sign of agreeing on a conclusion based on gut feelings and without verifying if the arguments leading up to them hold water.
Which claim are you referring to here?
1. Your unsubstantiated claim of ADHD being overdiagnosed
2. GP's unsubstantiated claim of "Every X knows Y"
The null hypothesis must be "ADHD diagnostic criteria has high specificity", because without that it's not diagnosis. What evidence do you have to demonstrate low specificity?
That smells a lot more like under-diagnosis.
I personally think definitely there’s something to it.