I've read everything that Alicorn wrote in that thread four times now, and don't see anywhere where she said to assume that the other person has started the conversation. She didn't give explicit advice on how to start a conversation, but note that the original comment is marked "some tidbits", not "everything you need to know about having a conversation".

She does give a useful, if rough, heuristic for determining when one shouldn't try to start a conversation with a stranger:

If you do not have practice using social spontaneity to good effect it is ideal to try it in situations where the other party may both physically and socially escape, just as a general rule.

Further, auties can learn heuristics that mitigate some of our social skills deficits, and Alicorn's advice is generally within the realm of such heuristics; she doesn't suggest reading the other person's body language, for example, but gives advice that is likely to work without the knowledge that body language gives. Also, as a strong extrovert, Alicorn is more likely than most auties to have developed those heuristics to the point where they can be built on to create more advanced heuristics that go well beyond what is stereotypically expected of an autie.