The number of members in the ruling party doesn't really mean anything, lots of people were members of the communist party in the Soviet Union as well, but it was still an autocracy because only one of them had any real power. China being called an autocracy is somewhat of a hold over from when Mao took over, the government still operates similarly to his outline, but the power structure is closer to an oligopoly rather than a true autocracy. Autocracy is still commonly used because more people understand it and its "scarier" than a word no one has heard of that sounds more like a cheesy board game knock off than a method of oppressive rule.
To more directly answer your question, following the strict definition of autocracy China is not an autocracy. However, they don't really fit with strict definitions of other terms for power structures either so autocracy is chosen as the most accurate term.