Straps can be trapped outside doors as described. The end of the strap had a metal/plastic fitting that will flap in the airflow and apart from the clattering noise, may potentially cause damage to the fuselage. It is a Cabin Crew check to ensure that the straps are not caught in the door frame. Many modern aircraft now have retractable torsion type safety straps in the frame in order to avoid this happening.
It is an immutable law that on any given passenger load, there will always be one barrack-room lawyer type old git as described above. The best response is usually to remain scrupulously polite, and to ensure that information is accurate and updates frequent - this will ensure that his fellow passengers will eventually tire of him and say all the things that you, as crew, just can't.