armored rider can fall and get up
The 'myth' comes from very late period jousting. The helm was often bolted to the breast thus preventing the rider from bending his neck. If you can not bend your neck, you can not get up.
Try it! Lay on the floor on your back, side or stomach. Now, without bending you neck in any direction, try to get up.
One more case of a specific being inappropriately expanded to encompass a whole.
Siobhan ni Seaghdha, OP