They do know about both. Afaik, Anki looks bad has better looking alternatives that allow you to memorize they have been excited over. Not sure what, never cared. Specifically Anki is just flashcards. It is boring rote memorization and not even effective way of learning. I think that comparatively few people will do it as a hobby.

It is not that they don't know alternatives. It is that Duolingo has different purpose and they like it better.