The driving distinction between capitalism and socialism are the following:
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Which class controls the state?
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Which form of social ownership forms the principal, ie rising and dominant, aspect of the economy?
The socialist market economy of China has, as I already demonstrated above, a working-class state with public ownership as the principal aspect. It thus falls squarely into how Marxists understand socialism. This is reflected by how the working classes are the ones seeing the primary focus on development and upliftment in China:
State capitalism, on the other hand, may have strong public ownership, but ultimately will be a capitalist dominated state. This is reflected in how state-owned industry ultimately is run in a manner that supports the capitalist class. Famous historical examples include Nazi Germany, as well as modern Singapore and the Republic of Korea (for at least the first few decades).
This is why Marxist analysis is enormously clarifying. Without understanding Marxism-Leninism and Mao Zedong Thought, it can be difficult to understand China's social structure, as well as why the majority of communists support China. If your understanding of socialism looks like a fully collectivized, horizontalist system, then anything that exists in real life gets measured against such a model. However, socialism is enormously dynamic, as it is a transition between capitalism and communism, and thus there is no "one" socialism, not even within a single socialist state!