The answer here seems to be a question.
So the question I have is this: should I leave a comment asking the poster to create a new question, or should I flag it so that a moderator can remove it?
You should do both. I say this because the flag will get a moderator involved, and the comment will tell the poster that they are doing it wrong and should instead post a question. This comment may actually show in the inbox even if the "answer" is deleted.
Any answer that is "Not an Answer" ("NaA") should be flagged for a mod to either turn it into a comment or delete it if it isn't suitable as a comment or answer (in this case, for example)
There are also cases where NaA items are actually spam and should be flagged instead for spam, however those cases are rare.
This "answer" that you identified, though, suffers from one additional, very huge flaw: it's a question that would not be on topic on Ask Ubuntu anyways.
Kali linux, referred to in their answer, is based off of Debian, and is a variant (aka "rebirth") of BackTrack.. Because BackTrack is not a supported derivative of Ubuntu, and Kali isn't even a derivative of Ubuntu, they are offtopic, as outlined in the Help Center items about what is or is not on topic here.
The individual should be directed to use Unix and Linux instead, and I have commented on both the "Post questions as new questions" and the "Kali linux is not supported on this site" aspect, and also flagged that "answer" as Not an Answer.