This question (which is currently closed as off-topic, and deleted) comes from a known troll account. They're currently suspended on two other sites for trolling, and their MSO account was nuked entirely for repeated trolling.
Believing that this is also a troll question, I flagged this post as Rude/Abusive. However, a moderator declined it, citing lack of evidence.
Why was that? Was I correct in flagging this?
That might have been me. If so, I thought I was responding to a flag on your comment. In other words, I thought that the OP flagged your comment as rude to him.
In hindsight, I see what happened, it was actually you flagging the question. And I apologize for that. Keep in mind, the question was already closed when you flagged.
This is a case where you should use a custom flag rather than a standard one. Rude/abusive doesn't convey the extra information you're mentioning here to the moderator handling the flag.
I've deleted the post now.