The DSM is ... well, it's a many-purpose compendium, but one of them is diagnosing people to get them a starting point. For insurance claims, for looking for specialists, etc. But it's certainly not in itself an expert manual. (While yes it is, but it's really not. Because it necessarily has to exclude the outlier cases, so it intentionally limits its own scope, focusing on the most common symptoms, and general descriptions of them. Which is very hard for psychological phenomena.)

And it was because it seems those are all caused by the same underlying biology, so they are the same disorder with differing levels of severity in their symptom categories. (Social/language and restricted repetitive behaviors.)