Actually, AC considers it neither. It is Transborder.
Canadian airlines have three categories - Domestic (within Canada), Transborder (Canada to US), and International (Canada to anywhere else). The distinction is important in terms of check-in/boarding times and which counters you use to check-in.
As it pertains to DME, you luggage arrives at MCO as if it came from a domestic US flight (since you pre-clear US customs/immigration at YYZ). If you tag it with the yellow DME tags when at YYZ, they will appear in your resort room (assuming that you are staying onsite) and you don't need to deal with them at MCO.
Note, however, that this doesn't hold true on the way home. You cannot use RAC or BAGS on the way home (and couldn't even if you were flying United, which is a participating airline OR connecting via a US city). You can take the DME back to the airport, but you'll have to take your luggage with you. You can also still do online check-in (or have the front desk/guest services do it for you.
