ADHD/Social Anxiety is close to autism. ADHD is related to autism; social anxiety is a common comorbid. So, whether you are autistic or whether ADHD/social anxiety is a better description, you will still have so much in common with autistics that many of the same things that work for autistic people will work for you; and you may find that you share more experiences with autistics than not.
Even if this doctor is right, you're a spectrum cousin at the very least--certainly not neurotypical, and probably with strong autistic traits.
I wouldn't worry about the specific diagnosis, so long as this label gets you what you need. Autism itself is not exactly a single entity--there are so many possible expressions of it, caused by so many different combinations of genes. So "Am I autistic" isn't quite the right question to ask... more, "How autistic am I?" For some people, not very; for others, like you, quite a lot. Only when autism is the best possible explanation do they diagnose it; but plenty of other disorders are related, share traits, and generally overlap.
They may have diagnosed you ADHD/social anxiety because all of your autistic traits could also be explained by that combination, and they prefer to diagnose those because they're more common and less global than a pervasive developmental disorder. If there's a strong anxiety component, you'd have gotten the social anxiety diagnosis on top of any PDD label anyway.
Whatever your diagnosis, you're still welcome here. There are plenty of people who aren't quite on the spectrum, but still have lots in common with people who are--people with NVLD, learning disabilities, schizoid personality, social phobia, selective mutism, ADHD, or speech/language disorders. No reason to exclude them. As a community, we don't have to police who gets to participate--we don't have to define ourselves along diagnostic boundaries.