Maybe you are right and it was something else. But, it was very similar and the timing is quite close.

I just feel to share it if it may help someone to build a timeline. And, as I have said in another comment, Osaka has many many Chinese visitors and I was in a very busy airport. I doubt that it took a lot of time to jump from China to other Asian countries.

I don’t think it’s impossible, unlikely given current information. Of course that information can change…

Impossible is a very strong word, there are samples of people from months before covid was identified in Wuhan that are under investigation. Not specifically in Japan, but it can't be ruled out with the current information.