The problem as stated was: "it's prohibitively expensive (from a payment processing perspective) to receive lots of small, sub-dollar payments", and "a lot of friction if you're relying on your users to pull out their credit card info for such a small amount".

These are solved. User adoption is a completely different issue that applies to almost all donation systems. Even if nobody uses X, it doesn't mean X doesn't exits.

You missed the parent's point which is that just because a product exists, doesn't mean a the problem it addresses is solved in practice. Even a very good solution needs to be known about, readily available, priced correctly, and not suppressed by other forces, as well as extant.