China doesn't want another Tiananmen Square type protest, the leaders are old enough to remember the 1989 massacre. Unfortunately they now have to go through the same pain as other countries did early on with the health system overwhelmed and a daily death total likely to be around 5000. Things could have been different had their vaccine been more effective and the vaccination rate higher.
Requiring tests from travellers originating in China simply ensures that the health system in the country of arrival won't be burdened with more sick people, COVID is now endemic worldwide and allowing Chinese travellers in won't make any difference, new varieties may appear and vaccines will be adjusted just like they are for the flu.