I've deleted the duplicate, but I can't really do more than speculate about why the flag was declined before, and offer some suggestions about how to flag this type of thing in the future.
As I currently understand it, the helpful/disputed flags are the result of actions taken (or not) by the mods, rather than something the mods set directly. So, if a mod doesn't take the action the flag calls for, it's marked "disputed."
If other mods understand the system better than I do, they may chime in. That's the best explanation I can provide right now, though.
As for the type of flag, I would say that "Not an Answer" isn't really the right flag for this because moderators looking at the flag might not understand why you think it's not an answer. Instead, I would have flagged it with "in need of moderator intervention" and then provided a detailed explanation (including a link to the duplicate/original answer).