In response to this question, the following moderator post notice was added:
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.
This prompted me to post a question to Meta about why automated responses to short questions should trigger such a message about answers.
As it turns out, this is not something that is automated, but something that is manually applied by a moderator. (And the text of which is questioned here.)
This leaves me asking: Why was this post notice left in response to this question? Are moderators not able to change the text of such messages? (In this case, answer should have been changed to question.) But perhaps more to the point, why was the original question deemed to be too short and, therefore, worthy of this (misleading) post text?
If the question wasn't clear, shouldn't there have been a more relevant comment made, or shouldn't it have been closed as "unclear what you're asking" instead?