It wanders deep into apocrypha, but the subtext is there even at the start of the better-known Critique of Hegel’s Philosophy of Right, which is where the “Opium of the People” line is derived from:

https://en.wikipedia.org/wiki/Critique_of_Hegel's_Philosophy_of_Right

The foundation of irreligious criticism is: Man makes religion, religion does not make man. Religion is, indeed, the self-consciousness and self-esteem of man who has either not yet won through to himself, or has already lost himself again. But man is no abstract being squatting outside the world. Man is the world of man – state, society. This state and this society produce religion, which is an inverted consciousness of the world, because they are an inverted world. Religion is the general theory of this world, its encyclopaedic compendium, its logic in popular form, its spiritual point d’honneur, its enthusiasm, its moral sanction, its solemn complement, and its universal basis of consolation and justification. It is the fantastic realization of the human essence since the human essence has not acquired any true reality. The struggle against religion is, therefore, indirectly the struggle against that world whose spiritual aroma is religion.

Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.

The abolition of religion as the illusory happiness of the people is the demand for their real happiness. To call on them to give up their illusions about their condition is to call on them to give up a condition that requires illusions. The criticism of religion is, therefore, in embryo, the criticism of that vale of tears of which religion is the halo.

The conditions were ideal, even exceptional, for the people of China to reject religion in their proletarian revolution. The people attained an early post-religious viewpoint on their own; they didn’t need, or even have use, for someone to approach them as they toiled and suffered pre-revolution and tell them why were, quoting this thread, “dumb” for what they believed. The revolution, as I said before, provided the post-religious societal movement as the will of the people, not some ideological conversion from some self-appointed luminary looking down on them from afar.

I’m curious how he’d explain countries like China being so anti-religion, in practice if not in rhetoric?

China is not remotely anti-religion in practice.