The descent profile has several segments one of them is repressurisation segment. When necessary this produces a repressurisation rate during descent. it is a function of the destination altitude and the selected cabin rate. During this the ATHR actually is behaving like in speed mode to maintain speed to a reduced rate of descent to adjust the cabin rate to a/c descent rate. So it is not at idle. After this segment starts the idle path segment when the ATHR stays at idle and AP manages speed.