The UDID embedded in receipts prevents/deters people from sharing them with others using MITM techniques. Sharing does happen and Apple's article helps address that issue.
True, but that is only one solution and a flawed one, as device ID can legitimately change - no developer needs to know whether I am using the phone I bought today or the one I bought a year ago.
Non-consumable in-app purchases are restorable on any iOS device you can sign in with your iTunes account. When you buy a new iOS device and use StoreKit's restore transaction feature, Apple will generate a new receipt with UDID of that device. In-app purchases are tied to your iTunes account whereas embedded UDIDs are tied to devices you sign in with your iTunes credential.