The only source of their stats is the party. It's part of their politics to downplay the bad and show off the good. They have nothing to gain by showing to their population that they are weak in front of this pandemic.
The remaining countries have embassies, counsel general offices, and similar throughout China. Beijing has 172 embassies plus missions from the EU, African Union, Arab League, and the UN.
24 other cities throughout China have counsel general offices [1].
All of these have extensive contact with the local populations, and all of them have secure means of communication back to the home governments.
There are a large number of foreigners from a wide variety of countries living in China for business reasons, or visiting China for business reasons, who have extensive contact with the local populations.
The party might be able to control the domestic flow of information sufficiently to keep their own people in the dark, but for events with wide impact such as epidemics and pandemics they can't stop other governments from finding out through reports from all those foreigners.
Conclusion: either China's death rates are actually quite low from COVID, or they are not and the other governments (including the US, all of the EU, Japan, Australia, Korea) know this but are keeping it secret themselves as are the numerous non-government workers who regular are in China.
[1] https://en.wikipedia.org/wiki/List_of_diplomatic_missions_in...
That's precisely what I'm talking about, right? That information got out; the world was able to see information indicating a higher death toll that official numbers implied, despite attempts to keep it quiet.