I agree with canadiancow, it has nothing to do with MCT. I connect on to YYZ-YOW flights that, because of delays on my inbound in to YYZ, are under the YYZ US/INTL-DOM MCT all the time - as in probably several times a month. If you're there at T-20 then your seat should still be there, and if not then it is IDB.

Normally things are fine, but for whatever reason I've had a handful of unpleasant experiences in these situations with AC offloading me even though I made it to the gate in plenty of time. I even had one earlier this year where I showed up before boarding had even started and AC had offloaded me. In those cases though I am pretty sure I have always had an email / app notification and/or a phone call from the concierge so I'm ready to insist I get put back on the plane.