A cill, located beneath your door frame, mainly functions to divert water away, preventing it from entering the house. It’s important to recognize that the size mentioned refers to the total projection from the back of the bi-fold frame, not the entire projection from the frame’s front.
You can find detailed cill technical drawings below, which provide further clarification on this matter.
It’s generally advised to have a cill for most doors to assist with drainage, even if the objective is to create a low walkthrough effect. For instance, external decking or paving can be extended right up to the cill’s edge.