It’s good to be cautious about nonprofit organizations, but in the case of DSI, they’ve been around a while, have a good reputation, and score well on third-party sites like Charity Navigator.

I’d also like to make clear that their Dark Sky Sanctuary certification isn’t a scientific one based solely on light pollution, but also that the local/state/etc governments have implemented certain policies to help ensure the area remains a dark sky area. It’s best to think of it akin to something like a designated “wilderness” or “wildlife sanctuary” area, but for starry skies. Because DSI works with governments to certify these areas, you’re right that certain regions are likely to be more represented, and some not represented at all due to geographic and political barriers.

P.S. I’m not affiliated with DSI, but have used their accreditations in the past to pick wilderness areas to visit for hiking/camping/photography.

They do accreditations internationally too, though you’re almost certainly right that some locations aren’t as represented, especially since it requires communication and cooperation with the governments for these areas.