And didn't they say that patient zero had covid in November and WHO says that there were no cases before December
That's an example of emerging science in action. Once people have realised that a handful of unusual cases of ARDS-type pnemonia are connected, and then have looked for, found and sequenced novel pathogen, they can then start going back through records of any other patients who presented similarly in recent past, and re-test any retained samples.
Thus is standard practice - if you look at MERS, it was originally believed to have emerged in Saudi Arabia in September 2012, but the restropective investigation of cases established it in Jordan in April 2012.
This is not because there is a cover up. It's simply because when there are only very small numbers of slightly odd cases, the possibility of a significant link by novel pathogen isn't the first thing that anyone would think of. But by the time you have several cases and are worried that you perhaps have a new outbreak of SARS, but it's not-quite-SARS, then klaxons might start up. It's entirely possible that, like MERS, there could be preceding cases before a big enough, odd enough cluster occurred