It does strike me as weird.

I think it's because you're celebrating that your kid got a diagnosis, and not the actual kid. It's more about celebrating that you are autistic. You also bring yourself up and how your brains are wired the same as if that something to celebrate.

What if they weren't autistic? Would you celebrate that?

I could of course be wrong. But to me it's odd. And I know that I would've hated being celebrated for my disorder, something that isn't really about me in a sense.

And now that you have associated yourself with autism there's a risk that any negative feelings about the disorder will create guilt. Furthermore, you've more or less decided for your child that this is positive news.