Even though you don’t know it, you probably have an app (or several), music or something else that transferred over from your old device that was originally downloaded or purchased using your old account. If you delete it, it should stop prompting. If you can’t figure out what it is, setting up as a new phone should work.
Darn, I didn’t see this one yet and I just made one too. At least they are different enough to stand on their own.