That's kind of what it means.

OK, so: Basically, autism is almost entirely genetic. Heritability is in the mid-90% range. So the genes affect how the brain develops, and the brain develops differently because you're autistic.

Most brain development is done at birth, so yeah, it's "wired in" at birth. But humans are essentially born premature because of our big heads needing to fit through a woman's pelvis, which they can only do while babies are still tiny and helpless... meaning that a big chunk of brain development takes place after birth. Most of that is done by age three, but the finishing touches aren't in place until your mid-twenties.

And in addition to that, the brain is really, really flexible, especially before birth. If it weren't, identical twins would have identical personalities--but they don't. Maybe quite similar, but they are individual people. So exactly how autism affects you can be determined while your brain is still developing. That high heritability is for all autism spectrum disorders; it's not at all uncommon to have one identical twin with one set of autistic traits, while the other twin has a distinct set of traits.

So because of how long the brain takes to develop, the term "neurodevelopmental disorder" means not just an atypical brain to start out with, but atypical development as you grow up. An autistic child might learn things "out of order", faster or slower or just unpredictably, or pick them up as savant skills, or do them in a way that seems totally counterintuitive to typical folks. Autism doesn't just start you out different; you grow differently, too. That's probably a good thing, because the brain is so flexible; it's likely that some of that atypical development is an attempt to compensate for the fact that you were born into a world that's not exactly brain-friendly for you. I think autistic kids adapt to the crazy mess of a world we live in pretty well, all things considered, especially given a good environment that gives plenty of opportunity for learning, but doesn't constantly overload them or push them past their limits.