I read recently a simple explanation, the energy that the aircraft is using to push itself through the air is being reduced as some of that energy is being used to spin the windmilling blades.
Another way to look at it is the potential drag energy 1m in front of either a stationary or windmilling jet engine is the same. You could say that if some of that potential drag energy is then used to windmill an engine then a windmilling engine has less drag than a stationary one, however, turbine blades are designed to push air backwards rather than be pushed by the incoming airflow so their may actually be no real benefit at all, in fact it could even be worse.
A lot would also depend on the engine size, design, location, airspeed, etc, an interesting question nonetheless.