The point is that just dumping an arbitrary amount of cold into the house is unlikely to change the temperature because the thermostat has access to more heat, and has a different goal.
That example was meant to illustrate why simple one off actions have a diminished or imperceptible effect on the system.
It's just not a great one because an open freezer or fridge will not cool a room at all, anyway: it pumps heat (with some inefficiency, i.e. more heat) from inside itself into the room, so it will on average heat up a room if it's left with its door open.