Because the plasterboard is protecting the floor structure, not necessarily the room above which is just a bonus. The idea is that the floor is protected for long enough to let anyone in the room above escape.
You may not need a specific fire or smoke sealed hatch, but the opening (the sides) would need protecting. You may also need early fire detection and warning. It's something that the council team would have to advise on as there may be different interpretations. You should also inform your insurer.
