No that is not exactly true. CX and Asiamiles uses the SAME computer reservation system. CX can access the Asiamiles reservation but they have been told by AsiaMiles that they CANNOT touch the reservation without an authorization from them. If they have to call Asiamiles on your behalf, that's what they have to do.
In addition, CX should be able to tell you what the toll-free number to call from Canada to Asiamiles in HK is.