Yes, you have identified the correct definition of liberal for this case. And yes, the usage is correct. Personally, I would have written towards rather than to, but that's a stylistic choice and doesn't change the meaning or grammar.
Let's examine the relevant grammar, in order to avoid confusion over what liberal is describing.
Therefore, we do not claim that our adoption of rules liberal to plaintiffs comports[,] necessarily, with some Platonic ideal of perfect justice.
In this case, we are seeing three nested descriptive clauses. To plaintiffs modifies liberal, liberal to plaintiffs modifies rules, and of rules ... plaintiffs modifies adoption.
What does it mean? The court has adopted some rules. Those rules are liberal, but towards whom? Plaintiffs; those who are suing in court. Putting it together, it means the court's current set of rules gives some degree of preference or favour to the rights or privileges of plaintiffs.
Why does liberal here mean favourable and not merely respectful? Because the context makes it clear that plaintiffs have some sort of advantage. Here is an abridged (and edited for grammaticality) version of the sentence which should make this clear.
We do not claim our rules comport with perfect justice.
Because the rules are not perfectly just and are also liberal towards plaintiffs, that means they are favourable rather than just respectful. This means that plaintiffs must be given some sort of leeway or overly broad consideration, in order to protect their rights.
For example, in the U.S. (West Virginia), people have the right or freedom to file a lawsuit and seek civil damages for almost any reason; by getting into a car accident and then suing the auto manufacturer, say. Even if the crash and subsequent damages were caused entirely by operator error and the claim is dismissed, the defendant must still spend time and money to fight the suit. In this way, the legal system provides an inherent advantage to plaintiffs: even if they don't recover any money, they still force the defendant to spend significant resources. As the quote states, this is not perfectly just.
If we substitute in the definition of liberal, we can see that everything fits together neatly.
Therefore, we do not claim that our adoption of rules favourable to the rights and freedoms of plaintiffs comports[,] necessarily, with some Platonic ideal of perfect justice.