The case presented is indeed a good example. But not of what you're trying to convey, sorry.
The reported answer states a correct fact and it's important to see the question. Once we see it, it becomes apparent that the answer is completely off, so the upvote was definitely out of place. However, this is not always the case. I have flagged posts as “not an answer”, but the flag ended up to be disputed.
Depending on the situation, a member of the community might find that an answer with only downvotes is not deserving it and upvote; I've done this a few times. Context is important, reading both the question and the answer is even more important. Whether a post is “not an answer” is subjective and your opinion is as good as anyone else's. You're not the ultimate judge, community and the moderators are.
If you deem a post to be “not an answer”, flag it, perhaps with a downvote, and move on. You can return to it and vote for deletion if it's still with only negative votes.