not defending those doctors in any way as they should keep up to date in their medical training, but we didn't even really have a proper definition of autism until ~1994 when the dsm-4 was released and the name "Autism Spectrum Disorder" wasn't used until the DSM-5 came out in 2013. The DSM-5 it's self isn't even a great definition as experts say that it's definition excludes more older people and women than the DSM-4 did.
this means that therapists who have not kept up with their learning, and have been practicing for decades may not have been told ANYTHING about autism, and the ones who stopped their learning 11+ years ago may have been trained on incorrect/bad information.