I'm not sure this is the reason. ORD-YYZ has been a carve out route since the beginning, and continues to be under the new agreement. UA and AC still put their respective codes on almost all, if not every flight between these two markets. Carve out doesn't mean they can't codeshare. It just means they can't coordinate on schedules and pricing. Also, IAH-YYZ/YYC are also on the carve out list, but listed as routes that will be codeshared.