What are the guidelines for flagging posts, again?
I'm not the moderator that handled your flag (they may choose to answer to give their exact reasoning) but the general guidance is in the SE FAQ:
How do I properly use the “Not an Answer” flag?
So, does the flag put the flagged post into the corresponding review queue, or not?
Yes, the flag pushes the post into the Low Quality Posts review queue (this can only happen once), but that does not mean a moderator won't handle it, especially if the community doesn't resolve it relatively quickly. By default, moderators don't see these flags for 15 minutes. In this case, there were only two (contradictory) community reviews in 22 hours, so a moderator made a decision.
I don't think this is a particularly clear-cut case. Certainly it might have been left as a comment - if the author had had the necessary reputation - but it also just about qualifies as a answer, in the strict sense.
It is certainly capable of being expanded into a fuller answer, and perhaps the user deserves that opportunity. For all I know, there's enough in that answer already to answer the question for the OP, so making it a comment would actually be counter-productive.
Perhaps it would have been better to leave a comment guiding the new user to improve their (partial) answer than simply flagging it NAA? New users are generally not fully familiar with how the Q & A format works, or what sort of answers we prefer. Flagging without voting or leaving a comment wasn't the most helpful option available.