Because even though it has a question mark, it is an attempt to answer, right?
Seriously, if they are making a suggestion, it's an answer, even in the form of a question. If you don't think it's a good answer, down-vote it. If you don't feel strongly enough to give up 1 rep point to do so, move on. Don't flag answers as not an answer just because they're not in a form you expect. Arguably this could have been a comment, and I often leave comments like that to seek clarification on what they've tried, but in this case it could also be seen as a potential solution.
Related: Why was this flag disputed?
Also see all of the related questions to the right -------->
Note that, in some cases, because of the current moderator workflow for processing flags, valid flags have to be declined in one fell swoop along with invalid flags. So let's say someone flagged that as spam, which is clearly wrong, in order for the moderator to get the right message to that user - that they flagged incorrectly - they also have to dismiss yours. See @Shog9's answer here for more information, and also feel relieved that this is getting fixed.