FWIW, I think the "rule" is way over-played.
It is IMPORTANT to note that a guy with a contact weapon can close significant distance quickly... You shouldn't feel safe just because the guy can't reach you RIGHT AT THIS INSTANCE. Learn that, and move on.... 21', 22.5', 18'.... who cares? Understanding the principle is much more important than any specific test/demonstration/detail.
Once you understand the principle, you can develop techniques to help you deal with it:
- Move Offline while presenting your firearm.
- Move perpendicular to the line of attack, not backwards.
- Act as Early as possible to prepare to defend yourself, regardless of distance and immediate lethality.
Honestly, very few people get attacked with contact weapons from further than 1 or 2 arms reach anyway... learning to fight inside of 5' is much more important than gaming what you will do against a guy confronting with you with a knife from across the parking lot.
-RJP