If the goal was really to sell educational materials, then yes, those 16,000 people are the vast majority of people who would ever buy intensive Lakota language instruction, and that's why he's refusing to donate.
There are a small number of linguists specializing in Native American languages, and probably an even smaller number of non-Lakota amateurs. For the rest of people, a YouTube video will satisfy their curiosity and they would never pay for a full course.
But it would be a dick move for them to spend all that time with a community for the purpose of preserving an endangered language, and then make it harder to preserve the language.
Bruh, did you even finish reading my comment? I know it's fucked up. But you were wondering why it'd be such a big deal to provide them with free materials when there are only 16,000 of them. And the answer is because they were always planning on selling it right back to them.