People with Down Syndrome, for the most part, are NT's. There's jus a part or two of their brain that's not working properly. Socially, they're NT.
Continuing, NT is used to mean non-autistic here because it fits well. Anyone here who isn't AS is most likely NT by previous definition. A person with Down Syndrome has no reason for posting here. Or a psychopath, now that I think about it. And so, unless someone specifies, if we say their NT, that most likely means that they don't have AS, and are in general, a normal person.
Hope that was a better explanation. 
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Hello.