The hold is designed with the assumption that the outbound led is flown up until the DME limit (or specified timing) for all CATs.
True. But there is nothing that prevents the hold being completed well within the maximum limits.
ie. 20d to 25d represents a 1 minute leg at something like 280Kt. At 120 Kt, one can safely fly a standard hold ( 1 minute legs) and not go anywhere near the limits.
Thus if the flight has continued outbound for the planned time but not yet reached the DME limit there is no requirement for the leg to be extended until the limit is reached.