Asperger's was not reliably diagnosable between healthcare workers trained to diagnose it. In other words, a diagnosis of Asperger's in someone's medical chart was not a reliable way of knowing if they had Asperger's.
This isn't really providing any clarity to the question, since you're simply restating your previous claim. How is a diagnosis of autism any more reliable?
As in it has a higher inter rater reliability. The statistical term of reliability that is used when describing the likelihood to reproduce a measure under similar conditions.
What matters here is not the general inter-rater reliability for all autism diagnoses, but only those for the patients which would have otherwise been described as having Asperger's, which can often present as a very mild disorder. Given that many Asperger's cases are mild, it is no surprise that there is low inter-rater reliability. Is there evidence that for this type of individual, there is a higher inter-rater reliability with the diagnosis of "autism"?