All current actually existing socialism is in a defensive stance. This is needed because of the history of bourgeois liberal democracies seeking to destabilize and collapse socialist states by any means necessary. Socialist states that didn't do a good enough job of defending themselves are no longer with us largely because of such actions. This is precisely why government censorship exists in China, to help maintain the social order and ensure stability of the socialist state. Stability is especially valued in China for historical reasons. Other AES countries like Vietnam also censor things like hate speech or misinformation to some extent, including jail time for the worst offenders.
For Tiananmen Square specifically, there's a huge misinformation campaign from Western countries about the events of the protest. If you go up to a random Westerner, show them the iconic photo of the man in front of the tank, and ask them what happened to the man, they will likely tell you that the man was run over. This is, of course, not what happened, and there's video evidence to this effect. But the misinformation in the West is hegemonic and entrenched, and if it got around in the PRC, it would take a lot of time and effort to debunk it and not everyone would believe you anyway. So, for the Chinese government, they prevent people from bringing it up more than absolutely necessary by censorship. It's not the ideal solution but it will have to do for now.
If that is enough to squick you out about MLism you should do some introspection about why that is. If you're truly committed to revolutionary politics you may find yourself needing to do much worse than censorship in the future. It is necessary for revolutionary movements to strike a balance between achieving their goals and avoiding the worst excesses of revolutions. As we know from history, this can be quite a difficult balance to strike, especially if the people have had a boot on their neck for a long time and are full of righteous anger. We need people like you in our movements to rein us in, but not so much that the revolution fails and all of it was for nothing.