Am I missing something?
Elsewhere in this thread is a link to a separate organization called "OpenWebDocs," which appears to be an outside consortium that contributes to MDN. So perhaps donating to them is approximately the same in that you're donating to a group who will then contribute time writing content.
Edit: Perhaps MDN is not a stand-along thing, and the way you help fund MDN directly is by donating to Mozilla? Can anyone clarify the relationship of these various parties?