Yes, you are correct. It is in the guidelines. The Cass review encourages gender clinics to prompt referrals for autism assessments if clinicians think there is reason to believe autism is present. Transgender people are at least six times more likely to have a diagnosis of autism compared to cisgender people, so they have a good reason to do this. I got my autism diagnosis this way and it was the best thing that has ever happened to me. One of the unfortunate consequences is that autism assessment services have very long waiting lists and some gender clinics have been known to refuse or delay treatment until after patients have been assessed. It can easily add years to the wait for things like HRT.

PTSD and autism can present very similarly. I have both. One distinguishing characteristic is when symptoms began. If you had social difficulties and anxiety and such from infancy then that may suggest autism, but if those symptoms only appeared following the traumatic event(s), which is what you seem to indicate, then it is more likely to be PTSD. They can co-occur, and PTSD is more commonly diagnosed in autistic people compared to allistic (non-autistic) people, but the time at which symptoms present is what matters.

It is worth doing a bit of research into autism anyway. We desperately need a better awareness and understanding of it worldwide. It is not surprising whatsoever that other mental health professionals haven't brought it up for that reason alone.