Again, I apologize for the off-topic (or at least tangentially related to the topic) post.
You must understand the world of 1918. We didn't have cheap, rapid, mass transit. People didn't routinely move about the globe (well, at least they didn't commonly before a World War started). China wasn't a locus of world commerce at the time. It was quite possible for a virus to circulate locally but not globally at that time.
I threw out the example as an instance where the starting date of a pandemic has been pushed back as more information develops. And indeed, no matter where the 1918 pandemic started, the consensus as to the when is around 1915 or 1916, several years before when it was noticed.
Let's take something more recent. The HIV epidemic was thought to have sprung out of the blue in the 1980s. Then a doctor remembered a mysterious illness of a sailor that he treated back in the late 1960s. It was such an unusual case that he actually preserved a blood sample. When it was tested, lo and behold that patent was found to have been infected with HIV circa 1968 (if memory serves). Gene sequencing of the virus, determining it's rate of mutation, and comparing it to the primate version of HIV from which it diverged kept pushing back the likely jump date to humans further and further back. I believe I've seen estimates of1920 to the late 1800s. And this for an epidemic in more or less modern times (trust me - 40 years isn't that long).
My point is, it is very common to see a pandemic suddenly appear and believe that you know the start date, but to later have that date pushed back as more information develops. We've already seen some of that on the arrival date in the US. Based on past history of these things, it would be normal to see it pushed further back both in China and in the US. It's possible that covid-19 is the exception, but that pattern is the rule.