Seized jet engines definitely create more drag than windmilling. Think of the windmilling engine as offering less resistance to the passage of air; it only extracts enough energy from the airflow to overcome rotational friction (= not a lot at windmilling RPM). The seized engine effectively presents a flat plate to the airflow, so while there is no work done on the shaft, there is plenty done on the whole airframe by the intake-sized airbrake thus ‘extended’. I’ve suffered both types of failure (fortunately in twin-engined aircraft) and the difference in thrust requirement from the remaining engine was very noticeable.