Why was this "not constructive" flag declined? I don't think that it contributes anything at all to the question and answers.
Did I perhaps flag some adjacent comment by accident?
Comments are, really, the least important things on the site. Flags on comments are left up to moderator discretion. We try not to be the Fun Police, so if you're having a short, civil conversation in comments, even if it's only tangentially related to the post, we're not going to delete it.
Still, comments aren't the place for extended conversations, or things that aren't at all related to the topic, and if there get to be too many we'd still probably want to clean it up.
In this case, there's no pressing reason to delete the comment. Sure, it's technically not constructive, but it's been around for almost 4 years and there hasn't been any issue with it. It could stay or leave and, aside from this meta topic, no one would ever notice.
Comments usually have a bit more leeway than questions and answers when it comes to flagging. The comment you flagged seems to be part of a conversation (and a short one at that, so not grounds for migrating to a private chat) and isn't offensive or derogatory, so we usually leave those be.
"Not constructive" is usually used for comments that are spam or spam-like in nature ("+1" or "first", for example).