Disclaimer: This was originally meant as a comment, however it got far to many letters for that, so I wrote it as an answer instead.
First let me say, I respect your decision, however I do not agree with it. I wish this would have been handled differently. Anyways, what is done is done. It's in the past and cannot be undone, so let us focus on the future instead, and improve on how this will be handled in the future. So please look at this as constructive critique, as that is my intention.
Going by your own saying:
What this post would normally receive on its own is 8 "not an answer" flags, 6 "very low quality" flags and perhaps a couple "moderator attention" flags, when the community could very simply and easily have deleted the post on its own through half as many spam or offensive flags.
It's clearly obvious that a lot of us see the flag reasoning differently than you did. And the flags should not have been declined. Instead you should have brought this up on the meta, since there clearly is a dispute of our views.
A reason for this might be due to poor wording of the offensive flag description:
This question contains content that a reasonable person would consider offensive, abusive, or hate speech.
As Tim Stone mentioned in a comment somewhere: Since the "abusive" part is smashed in the middle of offensive and hate speech. I would not consider gibberish abusive. But rather something more offensive than just gibberish. What that might be I have no idea.
However, now that it is explained to me, I can see the error in my reasoning. And I'm sure there are more of us seeing it this way.
As you said in a comment:
Again, that's been an exceptional behavior for an exceptional case. Stop worrying about it.
This cannot happen, and frankly should never happen. There shouldn't be any exceptional cases for flagging. As it will only make us more confused about what to flag as, and would likely make us flag less. Additionally, I have always flagged cases like this as "Not an answer", as I found it the most suitable flag, and it has always been approved by mods. When suddenly one of these gets declined, it would only make me more "afraid" of flagging, as I don't want to lose more of my flag weight.
Anna Lear brings up a good point as well:
If flagging as spam or offensive is gonna be policy, mods have to take extra care to dismiss appropriately since those special flags don't auto-dismiss as helpful when the answer is deleted like other flags do
Flag weight is there to reward us for appropriate flagging, so what is the point of flagging something we won't get rewarded for, when there is a reward system implemented? Yes, it is to help the site. But honestly, when there is a reward system in place for it, I would like to get that reward when flagging appropriately.
So we either need to enforce a rule that posts like this should be flagged with a red flag, or let it be fine with a non red flag. If it's the latter, this should never happen again.
You should have explained to us that we were using the wrong flag, before handling the situation like you did. And we should make this clearer for the future. Should flags like this be declined in the future, or allowed?