Does that mean if the law says the police have arbitrary discretion to decide who to imprison, then the outcomes align with the law as written and you still have rule of law?

How about if the law is written so strictly that everything is an offense, such that it's impossible to enforce on everyone, and you choose to not enforce it against people you like? Any case where it is enforced, you can still point to the law and say the outcome aligned with it.

And what if the law unequivocally says a certain group of people are slaves, is this justified simply because the outcome is not arbitrary and there is a codified rule of law? Whether the law is right or wrong does matter.

The relevant metric is whether you can reliably predict the outcome of almost any interaction with the government. If government actors are constrained by the law, you can reasonably predict what they will do. If the law is just a fig leaf, you won't be able to predict their behavior. No society is entirely autocratic or entirely governed by the rule of law, but they tend to cluster at one end or the other.

If I can predict that almost any interaction with the government will end in my destruction, that is not a sufficient standard of justice. The problem is not that chinese citizens can't predict the behavior of their government.

Rule of law is necessary, not sufficient, for justice. You cannot have justice if you do not know what behavior will get you punished, and you cannot know what behavior will get you punished if the government can act arbitrarily with impunity.

Edit in reply to below: the issue in China is precisely that the government's response to criticism is unpredictable. If Xi Jinping criticizes Chinese policy, that means a new policy is now in place. If a random guy on the street criticizes Chinese policy... maybe he'll be fine? Or maybe not. Depending on where and to whom he says it and how nice public officials are feeling that day.

And the problem with China is not that it's lacking rule of law, but that the laws are unjust. It is easy to predict what will happen to you if you criticize the government.