I believe it to be on-topic, and a reasonable* wiki contribution that benefits the site overall.
My own assessment is that the contributor faced a real-world problem (parallel
SELECT INTO not working as expected), found the answer, and decided to contribute that experience. It may not always be easy to judge the background and intention in such cases, but the user in question has a good history of high quality contributions to this site.
The self-answered aspect is a red herring. A good (or at least reasonable) question and/or answer that meets the site guidelines is welcome regardless of who provided it. In any case, Stack Exchange sites explicitly encourage good quality self-answered questions. That link is even featured on the Ask Question page:
*The existing question and answer do leave scope for improvement.
Very basic questions with an answer easily located in official documentation are likely to be closed (probably as too localized) by the community.
I don't believe any special measures need to be applied to account for the "cherry picking" scenario. The normal voting, closing, and flagging mechanisms should handle this perfectly well, should it ever occur.