Maybe the visual part of the mask donning demo should be changed?
As has been correctly pointed out, you do not need to have the mask completely or tightly over both your mouth and nose. The mask is only producing oxygen to add to the ambient air the user is breathing in the cabin which is now at the same altitude as the aircraft. There are many pilots and passengers doing the same in high performance unpressurised singles and twins every day using nasal canulas - for a considerably longer duration than would be required in the emergency that occurred here.