Wing down, top rudder ... isn't that a sideslip ? Sure the aircraft knows naught about crosswind on the runway but it sure can see slip when the wind comes in off the nose.
As we would all be aware, not all instructors know a great deal about the detail that which they purport to teach. In respect of a few I have observed, one might go to the extreme of suggesting they know approximately naught about their trade at all. Caveat - the great majority of instructors are very knowledgeable - it's just a matter of sorting the wheat from the chaff.
The US folk further confuse the issue by referring to "forward slips" when sideslipping while keeping aligned with a geographical feature, typical the runway during approach and landing.
So far as the instructor's comment is concerned, it is arrant nonsense, of course. The easiest practical way to demonstrate this is to incorporate a simple yaw string on the cowling or window. The abrupt shift in alignment when slip is introduced cannot pass unobserved. Much easier on a twin, of course. If a single, one would need to keep a constant, low power setting so that propeller flow doesn't mask the slip.
Furthermore, one of the work up exercises when determing aircraft crosswind limits is to check out the aircraft's sideslip capability at altitude.
Are you sure he wasn't just having the group on to see if anyone picked up on the deliberate mistake in the lecture ?
Tinny, do bring us all back a bottle or two from the meal.