Information about warm sector depressions can be found in the “Handbook of Aviation Meteorology” published in 1971 by her majesty’s stationary office. I have no idea if it’s actually reliable or factual, and I didn’t really understand it forty years ago either
A small section of the million or so words of text states that “The depression itself, travelling with the fronts, usually has a velocity roughly equal in speed and direction to that of the geostrophic wind in the warm air”, which seems to be pretty much what it says in the extract in the original post.
Don’t know if that helps, but they obviously exist somewhere in the collective memory of the CAA question setters.