Perhaps part of the confusion is the term demountable. Webster Defined as: to remove from a mounted position. I suspect the hinge is considered demountable because it is easily removed (demounted) from the cabinet and door with just 2 counter sunk set screws (screw vs bolt, I will address that later).
The hinge design allows for super fast adjustment of a overlay door in all directions. The cabinet maker has the extra burden of slot cutting the frame and the door. The hinge is attached by insert a plate into the slot made in the frame and door. Although the head of the set screw has a counter sunk Phillips head, it is actually a male machined bolt that marries into the female tapped plate. The set screw is not removable from the hinge, so no screws get lost. No screws are dropped, lost or stripped. The plate does detach, so beware. Technically the "set screw," is not a screw, but a bolt. It does not screw into the face frame or the door. Using a screw driver, the threaded bolt is turned and the threaded plate is drawn towards the hinge, pinching the face frame or door. The frame/door is "pinched" between the slot and the back of the frame/door. The pinching mechanism can be released easily to make minor adjustments to the door to achieve proper overlay of the door.
The difference between single and double demountable hinges is that a single DOES screw into the face frame with wood screws, because there is no slot cut. The single demountable still uses the pinch mechanism on the door.
Wood screws not required or even functional for a double demountable hinge. A screw driver is necessary to turn the bolt and make the adjustments to the hinge.