The disease might have already been global at that point, it doesn't exactly have conspicuous symptoms. Wuhan was a place where somebody thought of looking for anything unusual.
If the disease had already been global, then retrospective testing of wastewater samples, nasal swabs obtained for other purposes like the Seattle Flu Study, banked blood, and other stored samples would have revealed that. There were some scattered claims that it did, but none that held up very well under scrutiny.