If Streeting thinks this, it would be important to research and understand why. It's much too simple and frankly unhelpful to say it's an over diagnosis. For example, if the incidence of autism has increased, claiming it's over diagnosed might suppress other relevant factors, such as the effect of increasing maternal age, which could be reversed or researched further.
So Streeting would have to initiate a research project that looked at all the diagnoses that have been made within a timeline and then work out whether they followed or departed from diagnostic guidelines. The researchers would also have to reassess the patient and see whether the diagnosis was correct or not.
The impact of national issues on mental health, such as covid or the cost of living crisis would also need to be evaluated.
Has he done this? I don't think so. He's talking out of his backside.