China, while allowing explicit criticism of the government, usually censors and prohibits critiques specifically calling for an organized protest movement. (see this study for a more scholarly account of that) I imagine that this student, in calling for a climate strike without working through government channels, has triggered that requirement and is thereby by "censored" in a way. Agree or disagree with the tactic as you will, but it's how the CPC maintains control. By clamping down on protest movements that act outside normal channels, they hope to cut off avenues for movements to be co-opted by the Americans and other forces that would see China balkanized. The CPC is very happy for you to engage in criticism of their policies, so long as you do so from within the party and government apparatus. Working outside that apparatus is off limits, for better or worse.