You flagged the post as offensive where there was nothing remotely offensive in the question. This faq shows when it is appropriate to use the offensive flag:
Even if a post is a bad post for some reason or other, it is probably not offensive. The Offensive flag is meant to be used only in extreme cases, like hate speech, or abuse.
For example, if a user posts obscene images to the site, that should be flagged as offensive. But if someone says something bad about your favorite technology, that probably doesn't apply.
As a rule of thumb, if you can't justify something as being hate speech, or abuse, you shouldn't mark the post as offensive. Instead, you should down-vote the post.
When you decide to flag a post Offensive, you will get a warning dialog. Take this time to decide if the post is really offensive.
You had already flagged for ♦ moderator attention with a custom reason as off topic, which was marked as helpful, it was simply the offensive flag that was incorrect and superfluous.
It may save you time in future to know that there is a pre-written flag reason for off-topic questions:
- It doesn't belong here, or is duplicate
- Off Topic