Statistically...one would think of this massive occurrence in the past forty years...but if you look across at various examples in the 1800s...I think it’s just that people labeled the kids in those days as just “slow” and didn’t make a big deal about it. If you ever look over the personality traits of Kit Carson...you’d think long and hard that he showed autistic traits.
I believe you may have hit a major point, it isn’t that it wasn’t prevalent, it just wasn’t tracked as a unique condition.