Sorry for raising this up weeks later, because I just checked my flag logs today.
My spam flag on this answer was
declined - a moderator reviewed your flag, but found no evidence to support it
(Note: Post is already deleted and user nuked)
According to metasmoke, the post content was:
As per this FAQ on Meta Stack Exchange, rude or abusive flags are appropriate on this kind of garbage. Quoting below:
What makes something rude or abusive and when should I flag it?
Abuse of the system or community is everything that is created with the intention to harm them. This includes posts that contain no useful content at all – i.e. gibberish posts along the lines of:
Furthermore, as stated by Shog9, it's fine to use either kind of red flags for this:
... and the same applies to Spam and Rude/Offensive (source)
(it is also supported by a previous CM, Pops)
There's also another consensus on Meta Stack Overflow that mixing spam flags and R/A flags are OK, since they're both high-priority red flags.
Synthesizing all references presented above, I think my spam flag is appropriate on that answer, so why is it declined?
I cast a spam flag instead of a R/A flag because it is more convenient with Tampermonkey userscripts that I have installed.
P.S. There was a similar complaint by someone else about a similar issue over on Drupal Answers.