I’ve only seen bypass valves that block off one of the two heater core hoses to prevent flow but not both. Same idea as a thermostat blocks only one side of your radiator to prevent flow. So even though coolant isn’t flowing, it is heating up and pressurizing. There may be vehicles out there with an unusual design that blocks both inlet and outlet hoses to the heater core. But this isnt one of them.
Not trying to argue, just trying to share some of my knowledge as a former Ford tech.