They can be both tools and limits. If you only have a certain collection of tools at your disposal, there are certain things you won't be able to do no matter your level of mastery with what's available.

If your argument revolves around maximizing the depth and breadth of what you can accomplish, it seems like mastery is the thing, and rules are indeed limiting factors. One's lack of mastery is what imposes the extra limits on top of those imposed by the rules.