An autopsy is an exam that is performed postmortem. And that’s really how people are using postmortem as a noun for the analysis that they perform after the “death” of a company, so it is a synonym at least approximately, if you don’t see that I guess you don’t have a very good imagination.
replyTry and find me some sentences out there on the internet in which one of the words could be reasonably substituted for the other one.
replyThey don't mean similar things and they aren't used similarly. A postmortem is, as you note in part, a report or a meeting for the purpose of producing or discussing such a report; an autopsy is a surgical procedure. They're as "synonymous" as the words "cigarette" and "cancer".