Stack Exchange encourages self-answered questions, but they are normally questions about a problem for which users were searching a solution/answer they finally found. As finding the answer was not so easy, users can post the question and the answer because they think it could help other users, avoiding they repeat the same research they did.
The FAQ says:
You should only ask practical, answerable questions based on actual problems that you face.
I take it as meaning not to ask questions merely for asking questions, which also means not asking questions merely for the fact they have been found on other sites. Otherwise, it would be quite easy to fill the site with many questions.
In this case, it doesn't seem the question was prompted by a practical problem the user was trying to resolve. The user is showing the answer he found for a request done from somebody else.
To make a comparison, it would be like if I go to some Drupal forums looking for questions asked from some users, I find a solution for some of the questions, and I post both the question, and the answer on Drupal Answers.
It would be unfair, as I am posting a question for which I already know the answer, while other users should look for a solution.
As I am writing both the answer and the question, they should be exceptionally good. I should not write a question for which the answer I give is simply "It is not possible," or "There isn't such a word."
Suppose the words for which the OED is looking the etymology are 20. Is that user going to write 20 questions for which he immediately adds his own answer without giving time to other users to look for an answer? If that is the case, it is not even a challenge; it's a monologue the user does with himself.