It's not so much a pressure relief system, it's a weep hole to let you know when the wet seal is blown on the bearing. Coolant can drip or gush out the hole, depending on the failure magnitude.You are correct. It is there to relieve pressure when the seal starts to leak. By the corrosion in those pictures it's been leaking for a while.
It prevents the pressure of the coolant in the engine(15 PSI or so) from forcing the leaking coolant through the bearings and accelerating the failure.It's not so much a pressure relief system, it's a weep hole to let you know when the wet seal is blown on the bearing. Coolant can drip or gush out the hole, depending on the failure magnitude.