It's donation driven. Not sure if you were implying this, but I wouldn't call that unsustainable. Wikipedia, Internet Archive, and many other nonprofits are donation driven and have been around for decades.
Edit: I just watched their 2023 summary video, and it seems like they might also be doing more stuff with the plastic they extract that can be recycled/repurposed. That might be a potential revenue source, but not sure.
If they started charging for it, that would probably go a long way to end all cooperation with state authorities, very quickly. As in, have their right to deploy/maintain the retainers revoked, etc.
If those countries had the political will and financial resources to deal with the waste, they wouldn't need for an NGO to do it in the first place.
Edit: I just watched their 2023 summary video, and it seems like they might also be doing more stuff with the plastic they extract that can be recycled/repurposed. That might be a potential revenue source, but not sure.