That single behaviour doesn't make a person have AS or any other disorder. A person has AS if they have a several clinically significant behaviours/symptoms - meaning, well, sort of very "abnormal", "weird", "special" symptoms that you don't see as extreme in most other people - that impair them in the normal life they lead every day.
Not talking with others if you think that you've got nothing important to say to others can be one behaviour that people with AS can have. Sharing an exciting story, talking about how your day was, asking someone else about the bad day they're having, showing of your new *thing* that you just bought, trying to cheer someone up, making small talk to put another at ease/prove to them you don't ignore them and to prove that you are interested in them/don't hate them, thinking it's more interesting to join a conversation than to remain silent - if someone with AS rarely does these things, it can be a direct symptom of their AS and a way their social impairment shows.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett