Firstly, he estimates 99.9% of self-diagnosed aspies really are on the spectrum (the .1% who aren't, he claims, are typically claiming to be AS because the condition they really have carries greater stigma). He says at first he felt it was just fine to be self-diagnosed, and gives an example of a self-diagnosed aspie he knows who is doing quite well. But then he noticed in his support groups that despite his 99.9% comment, repeated frequently, self-diagnosed aspies seemed much more likely to think that he doubted they were autistic. And this made him think that most aspies probably need a psychologist's confirmation in order to feel secure in their aspie identity.
This made me think. I've actually had two separate experiences of self-diagnosis on the autism spectrum. With the first one, the self-identification as autistic, my experiences are a lot like he says - I called myself 'probably autistic' until I was officially diagnosed. And deep down inside, I kept wondering if maybe I was actually stupid and rude, as I'd believed before self-identifying as autistic. It was my diagnosis of PDD NOS that stopped that.
For my second self-diagnosis, of pathological demand avoidance, an official diagnosis is just not possible. The only center that diagnoses PDA is in England and only sees children under 16 years old. When I emailed the author of the original description of PDA, she wouldn't even give me her opinion about whether I had PDA, based on my description of myself. So no official diagnosis. But now I don't really doubt that I'm PDA. And what happened to do that, is that my mother read the description of PDA and told me it sounded just like me. So, in essence, my mother diagnosed me.
I've heard autistics refer to themselves as 'self-diagnosed and peer-confirmed', meaning that they self-diagnosed, then met other autistics who agreed with their self-diagnosis. My self-diagnosis of PDA is a bit different, because my mother's not autistic, but it's the same general idea - a non-professional confirming a self-diagnosis. It seems to me that maybe what's needed, for many people at least, is just someone else agreeing with their self-diagnosis. That other person needs to be seen as knowing enough to make that judgment, and as someone who'd be willing to say straight out if they didn't think you really fell into that category, but they need not have a degree.
I also wonder if this uncertainty about self-diagnosis is itself an effect of how much the category of autism is 'owned' by professionals. Virtually all gay people are basically self-diagnosed, and it's rare to see such uncertainty among gays (granted, homosexuality is also much more easily defined, so that could be it instead).