Flipping that on its head, it could be said that no system is Capitalist either, as all Capitalist countries have some degree of a Public Sector. Marxism makes explicit acknowledgement of this fact, because it sees Modes of Production as which is dominant in an economy and which way it is moving towards, and in the PRC the economy is based on the Public Sector and strong central planning.

If we call any system that contains elements of the previous Mode of Production and elements of the next Mode of Production "mixed," every system becomes "mixed." Relying on purity for Socialism alone signifies Socialism as some unique utopia distinct from all previous Modes of Production, and not a phase in historical development like Marxists assert. Moreover, it leads to interesting conclusions. If we assert that any system with both markets and public ownership is "mixed," does that mean a country with 99% of the economy in the Private Sector is mixed? What about 51%? Rather than relying on percentages, Marxists rely on dominance and trajectory. In this manner, the PRC is undoubtedly Socialist, even if it has a long way to go to reach Communism.

As a side note, working conditions are frequently exaggerated, they are fairly standard for the global economy and have been rapidly improving.

What on Earth do you mean? First, the vast majority of China is on the internet, secondly there are non-internet methods of polling. Here's the source, and the relevant section for how they took the data:

The survey team set out to assess overall satisfaction levels with government among respondents from across the socioeconomic and geographic strata of China. “It is always a challenge to obtain a representative sample of the Chinese population, particularly from interior provinces,” said Turiel. “Our survey does not include migrant laborers, for example. But given the fact that the survey conducted in-person interviews with over 3,000 respondents per year in a purposive stratified sample, we are happy that the results include not just the coastal elites or large urban areas, but also poorer and less developed inland provinces.”

Emphasis mine.