The example you cite certainly does at least attempt to answer the question. It should not have been flagged, definitely not as "not an answer".
That said, the line between "not an answer" and "a very poor answer" can sometimes be hard to draw, especially in cases where the "answer" seems to be answering something other than the question asked.
For example, if a question asked, say:
"In how many different ways can $n$ distinct items be arranged in a circle?"
which of the following responses would you consider an "attempt to answer the question"?
"It's very cold here in Finland right now."
"Beetles are the most diverse taxonomic order among the insects."
"Every continuous function from $[a,b]$ to $\mathbb R$ is bounded."
"An $n$-element set has $2^n$ distinct subsets."
"This problem can be solved using elementary combinatorics."
"The number of permutations of an $n$-element set is $n!$."
(Please disregard the terseness of the responses, as I wanted to keep them short for this example.)
All of those statements are true. None of them answer the question, although the last one gets close. (The correct answer, of course, is $n!/n = (n-1)!$.) Arguably, though, all of them could be attempts to answer the question — some of them would just have to be very confused or misguided attempts, if so.
On the other hand, given the current phrasing of the "not an answer" close reason, I can easily see someone flagging at least any of the first four answers above as "not an answer", because they all appear to be attempts to answer some other question than the one that was asked. Sure, those of us "in the know" might agree that such answers would be better downvoted or flagged as "very low quality" instead, but it's hard to see an NAA flag on such an answer as wrong, given that the flag description seems to support it.
(Conversely, many people who are used to reading "not an answer" as "should've been a comment" might support an NAA flag on the second-to-last answer above, even though its hypothetical — and somewhat dismissive — author might well have felt that it was all the answer the question needed.)
The fundamental problem is that there's no clear consensus on what the "not an answer" flag actually should be used for. The flag dialog says one thing, folks on meta say another thing, or possibly several contradictory things. I've even seen ♦ moderators seriously suggest that the NAA flag should not be used at all: arguably, if you feel an answer should be deleted, it's better to flag it as "very low quality", whereas if you think it should be kept but turned into a comment, it's better to use the "other" option.