Marxists theory posits that states, are tools for class-rule. Also that there has to be a rupture of some sort for there to be a change in which class controls the state. If you agree that Mao installed a proletarian state, and China has not seen a rupture, then China still has a proletarian state.
If you look at the Soviet Union there was a rupture (a particularly brutal kind of neo-liberal shock doctrine) that ended the proletarian state, and replaced it with a bourgeois state.
If you compare China to other capitalist states, life for example India, there is clearly are large differences. Particularly the poverty reduction programs.
Also the official CCP line is that they are only in the process of building a socialist society. What they label as primary/preliminary stage of socialism.
Also to paraphrase Eric LI, China is not a capitalist because because there is no way a bunch of billionaires can control the Polit Buro, Capital does not have enshrined rights, it does not rise above political authority. https://www.youtube.com/watch?v=D3HCv2Sf_7Q
China does have a capitalist mode of production, but the strategical industries are in public hands, which means that the state can control the inputs and outputs of the private sector. This means that a neo-liberal counter revolution of the kind that overthrew the Social Democracies in western world is not possible.
Another aspect is that labour protections in china aren't as good as for example in the first world, but You have to consider that China is at another level of development and much less wealthy. The labour-union does seem to have some problems. But they might still get universal healthcare before the US.