We never know the number of undiagnosed, many may be just capable of pretending but suffering.
Neurodivergent doesn’t mean a failure at communication. That is a very reductive view of such a broad statement. Adding to that the concept of Neurodivergent covers a massive range of things. Someone who is ADHD faces different problems than someone with depression. So nearly everyone may be neurodivergent in some form but the average of communication falls within range of how we do now.
That’s not what he’s saying. He’s referring to neurotypicality as just being the standard that won a standards war, as in a format war like VHS vs Betamax or HD-DVD vs Blu-ray.
Neurodivergent people are not a homogeneous group. You can be neurodivergent in different ways and it influences communication in different ways. It is improbable that one of the neurodivergent subgroups is frequent enough to turn out to be an overall majority. But you’re not talking about majority, so what portion of the population would it have to be?