If compression is entering the coolant channels, wouldn't the radiator cap should open before it ruptures the upper tank?
Yes, but IF the line from the radiator to the overflow tank is obstructed OR the orifice in the overflow cap blocked then pressure could build up. Also air pockets in the system can generate steam which will build pressure very quickly.
But yes, a normally working radiator and overflow system should discharge and catch any coolant above about 20 lbs psi.