Absolute nonsense. Thousands of civilian, military, and law enforcement academies have run these type of drills and show the danger of someone with an edged weapon despite some distance between the suspect and offender. The behavior that you are demonstrating now, is typical of "experts" who often pop up on internet message boards. The anonymity and safety of the internet lets lots of people without a clue pretend to be an "expert" in everything from law to police and military tactics, despite the fact that they have no training, education, or experience in any of the above. It's an amazing thing to watch.
Meanwhile it remains the case that there's never been an independent scientific demonstration of the even the Tueller drill's most basic premises - No statistical justification for 21 feet as opposed to 25 or 17 or anything else. No representative sampling of fitness in the general population to tell if it takes the "average" knife guy 1 second or 3 seconds or 10 seconds to move. No controlled or replicable measurement of human reaction times, save simply running the drill with cops while already taking its underlying claims for granted. In short it's nothing more than anecdotal guesswork.
That observation is no matter of my anonymity or expertise (of which I choose not to volunteer or advertise though I am thoroughly confident that it well exceeds the typical GED-toting cop with a few hours at the academy and a "criminal justice" certification from ITT Tech). It's a simple and undisputed fact about the Tueller drill: it is NOT scientific. So treat it and use it as the rule of thumb that it is and that it was originally written by Officer Tueller to be. But don't try to pass it off as something it is not just so you can duck behind it to justify violent and reckless behavior that costs other human lives.