Did I use the wrong flag? (Not an answer) Or am I incorrect in
interpreting the back it up policy?
No, you did well. The answer is indeed not substantiated enough and the Not-An-Answer flag is the correct one to use in such a case.
As Em C mentioned in a comment, the problem lies in the people reviewing your flag.
I did not [leave a comment], no. I shall do now. However, according to the linked meta post, it seemed like it was preferable to quickly get the answer away as it would make other people think those kinds of answers were okay.
yes, removing the answer quickly is preferrable, but that does not exclude leaving a comment stating why the answer is being flagged/removed.
- the reviewers know why it was flagged and
- the author of the answer knows how to edit the answer properly.