As I understand it, the opposition to the TMT isn't a NIMBY issue: it's more about a historically ignored group trying to gain influence over issues it cares about.
Not really. As typically used "NIMBY" has a component of hypocrisy. Just not wanting something to happen is "opposition". "Not in my backyard" connotes the idea that someone supports some project in principle, but not when it affects them directly. "I like wind power, but not when it blocks the view of my cottage", "I like high density housing, but not if it affects my property values", "I like mass transit, but don't want a subway station in my neighborhood."
In this case, "I don't want another giant telescope on my culture's sacred mountain" isn't about astronomy at all, it's about the culture (and the larger perspective of indigenous interest in local resources, and respect for a pre-existing treaty that wasn't quite honored -- these folks aren't primitives or hippies, they know very well this is a political process in the real world).
The majority of the indigenous interests support the telescope. This is about a minority of a group trying to gain status at the expense of everyone's interests.