It was declined because that answer was attempting to answer the question, even if tangentially.
It was me that declined the flag but it looks like I fat fingered the decline selection and instead of
flags should not be used to indicate technical inaccuracies, or an altogether wrong answer I hit the generic
Your flag was simply "This answer has nothing to do with the question". Upon looking at the answer I could see some quite clear indication that it was indeed something to do with the question and it was attacking the problem from a different angle. Your flag was factually wrong and I declined it as such.
I felt that deleting something that was helpful to someone in the future that might be handling a similar problem was counterproductive.
If an answer is attempting in some way to answer the question then it is an answer. The fact that it is not 100% helpful to the original poster doesn't mean it might not be helpful to another user in future. We want to build a knowledgebase of solutions, that might help everyone with the same answer, not just OP.
If you feel that it is exceedingly low quality then we have a different flag for that.
Otherwise you can simply comment and downvote as necessary. Moderators should not be used to remove content you believe is wrong, the community has sufficient abilities to remove or police content that is just not completely helpful.
Moderators are intended as exception handlers and as such
If you see anything in the system that is evil, weird, or in any way exceptional and deserving of moderator attention for any reason…flag it! That’s the primary job of a moderator: to look at every flagged post, and take action if necessary.
A lot of the moderation work is mundane: deleting obvious spam, closing blatantly off-topic questions, and culling some of the worst-rated posts on the site. The ideal moderator does as little as possible, but those little actions may be powerful, visible, and highly concentrated.
None of that page mentions deciding the relative level of wrong-ness needed to delete something that is trying to be helpful in some tangential way.