I can see two potential ways this question may be marked as "unanswered".
The asker downvoted it
We can not assume an answer does not resolve the question, if left un-accepted; however, we can assume that the answer does not resolve the question, if the asker then downvoted it. We can see that the answer has received one downvote, and the asker admits that it from them.
Hu, weird. Perhaps because I downvoted the answer? – Tim 3 hours ago
The account that posted the answer has been removed
We can see that the user account from which the answer was posted has been deactivated. It is entirely possible that answers from deleted accounts do not appear in the search for "questions with no upvoted answers", from which we derive the "Unanswered Questions" .
If it comes down to being downvoted by the original user, I would accept that this question qualifies as "unanswered". The downvote, itself, confirms that the answer is not useful to the asker. This would suggest that an alternate answer may be necessary.
If it comes down to the answering user deleting their account, this may likely be a bug that needs further investigation.