It is an attempt to answer the question. The author of that answer thought that he was answering the question. If that's the case, then NAA generally doesn't apply. It may be a poor answer, perhaps even Very Low Quality, to the point that it should be deleted, but it's not Not an Answer.
(That said, what to do with link only answers is generally a disputed topic that doesn't really have consensus among the community or even the moderators, although consensus has more or less confirmed at this point that NAA isn't appropriate, what should be done is still rather hotly debated.)
Now, there are some link only answers that are clearly not even an attempt to answer the question, so saying that you shouldn't ever flag them for this reason isn't correct, some really aren't even trying to answer the question, but most are, so generally this isn't the right flag type.