In that recent thread though, it wasn't a case of people trying to convince the poster he or she was not misdiagnosed it was more about asking why they were so sure they had been, given the obvious impairments they described. Any online discussion is always geared towards the counter argument, everyone joining in and saying 'I agree sounds like you have been misdiangosed' wouldn't really be a discussion, so people always ask the other to explain, and then play devils advocate back to them.
Other conditions get misdiagnosed, but from what I have read it is often the case that autism is the underlying cause at the end of the road of other misdiagnoses, where it finally makes sense. Probably can happen in the other direction but less likely, can't back that up with any figures but that is the sense I get.
This thread does seem really elitist, 'these fakers are encroaching on our autism' kind of attitude, it is such an internet thing, no one can possibly know if a stranger has it or not, to assume people are faking is based in cynicism rather than fact, id rather just respect that if someone has had enough challenges to get diagnosed or self diagnosed just to take it at face value.