Summary of what is known in this thread:
1.) The Chinese government is committing human rights abuses against members of the underground Catholic church and other groups. Nobody actually disputes this here.
2.) There is an official Catholic church in China that is currently not be subjected to human rights abuses. One person in this thread might not believe that but everybody else gets that.
3.) The Chinese government is run the by the Communist party. No one disputes this.
4.) The Chinese communist party is composed of Marxist Atheists. This is questionable.
A.) The current economic model followed in China is capitalistic one and is not compatible with Marxist ideology.
B.) The actual beliefs of the communist leadership are not something publicly stated. They may believe quite a few things. Until about ten years ago there were quite a few Falun Gong believers in the government there.
Given that the underground churches and the Falun Gong cult are being treated the same way but the sanctioned church is not, we can conclude that the issue is not one of belief but one of control. If the Government in China was actual interested in promoting atheism, they would also go after the official churches.
We can also conclude that since the Vatican has recognized the official church supported by government, the underground churches are not following the wishes of the Vatican and are actually heretics which are not following the instructions of the pope.
https://press.vatican.va/content/salastampa/en/bollettino/pubblico/2018/09/26/180926b.html