That would be correct, your wife can go as scheduled in IL. Have the biometrics appointment mailed to you and you can go to the nearest ASC in NC. If asked, explain that you are in NC on work or business and they will let you do your biometrics on the scheduled date. I have done this myself in a similar situation, during my AOS biometrics. The appointment was at the Oakland, CA office, but got my biometrics done in Charlotte, NC as I was in NC at that time for work.
You can do a search and perhaps find other folks having posted similar experiences.
I could have also suggested calling 1-800 USCIS customer support # to confirm, but if you get an agent who isn't knowledgeable they might end up asking you to re-schedule, which in my opinion isn't necessary.