Yes, this appears to be true. I just tried to order via the app and a pop up comes up saying mobile order at contempo is for guests only. Looks like that's the only restaurant that has that. Just tried GF and Poly and they allow mobile orders. I think if you are at the contempo and really need to eat, you can walk up and order but just like the other WDW restaurants, they are trying to avoid walk ups. If they did ask if you were staying on site, you could always monorail over to Poly or GF to eat. But in reality, I don't think anyone is missing anything, the food at Contempo isn't that great anyway.