I didn't decline that flag, but here's my take on it.
If you have the tools to do something as a user, you should be using those tools. There's nothing that I, as a moderator can do that you as a user cannot do. Therefore, don't put it into my flag queue. As an example, the right thing to do in this case would be to edit the topic title (along with the post and tags) to get it ready for migration, then vote to close as off-topic.
I disagree with Tim Post's suggestion of flagging if they are unable to ask questions on one site. You don't have that information, and even as a moderator, I can't see a user's status on another site. If the question is of sufficient quality to migrate and you can vote to migrate it to the right site, please vote to migrate to that site (the system will prevent questions from being migrated if they are banned from a site). If it's not of sufficient quality, it's highly likely that one of the other reasons will be a valid close reason, so vote for that or write a custom close reason. Of course, if it's a high quality question and you can't vote to migrate to the best site, please do flag it in that case.
Going back to the mod flag and putting it in my queue, if the mod flag queue is small that gives me more time to either answer questions or go through the review queue. Sending things to the review queue is preferable to sending them to the mod queue because there are far more people who can do things in the review queue than the mod queue. Save mod flags for things that are either extremely urgent or must be handled by a moderator.