I think they may have miscommunicated to you. If MX wasn't in OW when your ticket was issued then they will never be a valid airline for the ticket (the T&Cs are the T&Cs at the time you bought the ticket). The MEX-YVR is an exception because you had a confirmed reservation on JL on that segment and the flight has been withdrawn so QF is obliged to find some other means of getting you there. It happens to be on MX, but not because MX is valid on your ticket (it isn't) - the replacement flight could have been on any airline.