The Qantas situation states otherwise. In that case, passengers were on international journeys, but QF was still charged with illegal cabotage for operating the domestic leg, as the international leg was on another carrier (a code-share partner).

Qantas settled to avoid an administrative hearing, so we don't have an actual decision, but it's clear what the DOT's position was: illegal cabotage can occur on an international journey if a change of carrier is involved, because each carrier's actions are to be considered separately. Under that standard, there's no question that the AC flights IAH-YYZ-LAX would be considered illegal cabotage even though the passenger began his journey in Mexico on UA.