The reason borders were kept open was that it was considered xenophobic to close down the borders and globally public health guidance was saying that the virus was affecting only old, very young and people with weak immune systems. Only in the middle of March 2020 did countries start restricting travel and by then it was way too late.

Whatever the reason (many reasonable at the time), this was after PRC did a full lock down, with outbound flights limited to repatriation, i.e. they weren't trying to spread the disease, other countries wanted their citizens back while PRC did one of the harshest locked downs in modern history. Can't blame others for thinking CCP was over reacting, but ultimately, PRC locked down during period where covid spread in PRC was minimal and signifant first incidents in many countries were from western travellers, who thought things were going to be fine.