Start by looking at how IDB is defined in Canada. In the USA this would be IDB if and only if the flight were full.

To me, MCT is a restriction on what tickets/reservations can be booked and when flights must be rebooked in response to a schedule change in advance. If all passengers were offloaded whenever a delay in flight caused a MCT violation according to the new ETA, many many many tight connections would lead to offloading and forced rebooking, which would greatly increase the workload for airline agents at major airports. I consider it improper for airlines to do this unless/until the passenger has missed the boarding deadline, commonly T-15 for domestic flights.