I agree that insurgency is a strategy, not necessarily a description of a conflict. However, I don't think it's a strategy limited to weak organizations. Major powers have used insurgencies, especially through supporting proxies, to achieve strategic goals they could not achieve directly. This was one of the founding principles of Special Forces.

Perhaps you could argue that the system or environment the dominant power is trying to foster "dominates them" by imposing expectations on behavior (or to avoid perceptions of hypocrisy), leading them to take indirect action or work to undermine a rival in a way that avoids wider conflict.

Also, I don't know if this is a general characteristic of ALL insurgencies, but every one I can think of focuses on undermining the legitimacy of its interlocutor while trying to promote its own (or its objective).