Its funny how despite this the author is not even overtly rabidly critical of Marxism or Communism itself

Why would he - Liu Cixin has been pretty tight lipped about his politics but some of them were gleaned in one interview:

According to a June 2019 interview and profile article by The New Yorker, Liu avoids talking about politics. In the same article, Liu argued that democracy was not appropriate for modern China, and individual liberty and freedom of governance is "not what Chinese people care about", adding "If you were to loosen up the country a bit, the consequences would be terrifying." He expressed support for policies such as the one-child policy and the Xinjiang re-education camps, saying "the government is helping their economy and trying to lift them out of poverty". The article reported that Liu had "become wary of touting the geopolitical underpinnings of his work".

These few statements alone in a single article were enough to motivate five republican senators to write an official letter to Netflix asking if they were aware of it and that they shouldn't proceed with the netflix adaptation of the trilogy (they ignored them with a canned response). So one can see how he'd be wary of expressing political opinions publicly.