The relief well is just another well. I don’t think it will relieve the pressure substantially. Saudi Arabia is full of wells where the oil flows up and is collected using the pressure of the field. The fact there are lots of holes and wells doesn’t mean one will cease to flow when another is opened.
Eventually the pressure might be lessened but not in the immediate future.
The relief wells are not designed to relieve the pressure, they are planning on drilling into the original bore and diverting the oil to those two wells, they plan to seal this well from below once they have drilled into the bore.
The relief well is mainly to allow sealing of the main well from below. It will probably also relieve some pressure.