I found that this account was using an old email address that I no longer have, and when I tried to update that, it said that email address already had a forum account. So I reset its password and logged into it. It, too, said I had no game account...but when I went to look at the game account info in my settings, it already had the username for that account filled in. So I went to the "do your setup" link in the banner and clicked Next to dismiss the notification. That account appears to be in order now, so I've updated my password manager to make sure I use it. I'll see whether I can close this account so that there isn't a duplicate sitting around.
Thanks again!