We only 'know' that COVID-19 came out of Wuhan in December because China said so. The other thing is, before a test was available for it, no one officially had it. We have only been testing reliably where I work for just over two weeks.
I work with a guy who got a severe atypical viral pneumonia before Christmas. So did his girlfriend and her young daughter. All of them go to church a mile down the road from the Kirkland WA care home that had the big outbreak. Even he says now that he thinks they had it. We started seeing severe atypical pneumonia's at work before Christmas. That is what made us think he had picked it up from a patient. But now it looks like it was at Church. That being said, a lot of staff became very sick around the same time. Before Christmas. Management now plans to antibody test some of us that are in key clinical positions to see if we can relax our infection control regime slightly, as it has destroyed our productivity, and if any one of us go down it will severely affect our department.
If this is an emergent disease and not an accidental lab release, it is fair to assume that it has been percolating around China for quite some time. I have read research articles written around 2011 where the exact same bats were being studied, and concern was being expressed that long ago. I'm sure thousands of people die in China every day. I doubt a few thousand more here or there would get noticed unless someone was really paying attention.
There may have been small outbreaks of different strains going on for years that we would never know about, and all or a sudden we hit the jackpot with an especially virulent one that started jumping on planes and ships. Or, the Chinese knew all along, but didn't care to tell anyone. People die from pneumonia, and the 'flu', all the time. We have no idea when this one really got moving, but whenever it was, it had probably spread around the world within about 24 hours - as that is how long it takes to fly from China to anywhere else. That makes it November, or December at the very latest. If we believe the Chinese. I don't, and suspect that a milder version could have popped up at any time without us knowing.