This answer looks very much as not-an-answer to me but was declined (no evidence). On the bright side: it was at least deleted :-)
Screenshot so <10K can judge as well:
Any idea as to why my flag was declined?
Because certain things sort of stink, namely some of our moderator tools.
Your flag was valid, however a SPAM flag on the same post needed to be declined. Currently, we can't decline flags selectively (a new moderator interface in the works permits this, but it's a little while out).
Since validated quality related flags are used for review audits, I'm really sorry, but I had to invalidate that SPAM flag, which also now feeds our spam mitigation system as well. I couldn't validate it.
You didn't do anything wrong, just a quirk. We might be bolting some work-around scripts in place as a stop-gap until the new tools are ready for use.