In the eyes of the site, he had a question - and provided an answer! The course of action for this question is to have the author submit his answer, and accept it. This does happen (and it's usually happens to me nearly moments after I ask a question!) from time to time.
However - in regards to the idea of your question. If a question does appear that you think should be closed, and fits under any of those 6 bullets. It's not only your right - but as a community member it's practically a duty - to submit your vote. Questions will not be closed right away - it typically takes 5 votes to actually have a question closed - this is what helps keep the community clean.
As Moderators the four of us are merely here for guidance and do as little as possible in regards to interference. It's the members of this community who are really the ones that should "police" and maintain it's integrity. That's why (depending on your reputation level) more an more abilities are awarded (things like editing other posts, etc - which are all outlined on the FAQ - during the BETA this scale is adjusted though)
My last bit - as you've taken the right action. If a question seems inappropriate - and the 6 outlined "closed" functions don't seem to fit it - this (meta.ubuntu) is the best place to discuss it's course of action.