Let me ask one clarifying question here:
similar to OP, if I go as Plat for SFO-DFW-MEX-CUN, where MEX-CUN is not AA flight but bookable at aa.com- will I in this case get free access to AC because of considering DFW-MEX as international flight? I believe, rules say that everything in Mexico except MEX is to be considered as domestic, which indirectly says, that MEX is an international airport for the flights out of US.