Autism or well any form of neurodivergence are about how you work on the inside. It is not possible to observe how a person behaves and just diagnose someone. That is why getting a diagnosis is a whole process involving a trained professional.

Your colleague is full of shit. Generally, neurodivergence is for everyone who regularly experiences that the way their brain works causes them trouble.

Self diagnosis is surprisingly accurate but people also tend to under estimate the severity of their symptoms.