Here's a suggestion and no scientific evidence backing this.

The virus might have spread worldwide earlier. There might have been deaths due to it earlier too. There was no way of knowing it until Jan 2020. No testing. Nothing. Also the fact that this virus affects people with existing conditions, there's a very good chance that initial deaths / patient 0 / index patient might have been tagged as dead under their respective conditions. As far as the symptoms go, there's no 'unique' symptom so that might have led to people saying that their coughing is due to cold or whatever.