Are you confused or am I misreading your comment? The result is that 80% of positives are false positives, not that 80% of all tests are false positives. (IMO it is still fishy.)
I wasn't implying that 80% of all tests were false positives. I'm talking about P(You don't have coronavirus | You test positive). 80% is far beyond what would be normal for these types of medical diagnostic tests. I would expect something like 1% or less for a really good test, and around 10% for a bad one, but I don't know if the test the Chinese were using is really bad in some way, which is why I'm asking for clarification there.
But if your diagnostic criteria is showing symptoms later, then you are ejecting the entire population of carriers who might be, say, teeming with the virus but showing zero symptoms, for perhaps a genetic or "dumb luck" reason.