I believe this because I am a communist, I'm a member of a party and I study the works of Marx, Engels, Lenin, and the history of existing socialist states. In the case of China, the workers and peasants established political supremacy through a long civil war and revolution, solidifying a socialist state in 1949. Capitalists exist in China, but they do not form a coherent political body, the revolution decided that they will not be allowed to usurp the power of the workers.
Regarding communism, you're confusing the eradication of class society with the eradication of all centralized administration, and confusing the socialist stage with the classless and stateless communist stage. China has capitalists, correct, because China is a developing country. Communism is not a moral decree, but a mode of production predicated on immensely developed productive forces. China does not yet have those productive forces necessary for the complete eradication of class.
Regarding the economy of China, the commanding heights of the economy are overwhelmingly publicly owned. Private ownership is largely in secondary industries, and are utilized in order to accelerate the buildup of immense productive forces. This is called the Socialist Market Economy, which is a transitional form of economy in service of building more developed socialism.
To circle back, I do understand this. I work constantly through organizing work and study, both on my own and with comrades, to understand Marxist-Leninist theory and practice. It's critically important for my own work as a party member. It's critically important to not make any confusion on theory or practice, and to defend existing socialism.