Mental disabilities including conditions like autism and down syndrome are also straight up recorded throughout history. The entire issue is diagnosing someone who’s long gone and from a culture with completely different values and norms. The description of someone isn’t enough by itself.
For example, in Ancient Greece, Spartans killed children with mental or physical abnormalities. This could’ve easily included severe forms of autism. Autism could partially be an evolutionary quirk to make it easier for hunter gatherers to distinguish edible flora from poisonous ones. Even in medieval times, people with mental disabilities were used for entertainment, and people with severe autism and down syndrome (and even physical disabilities like dwarfism) were known as “natural jesters”