To me, a flanking move is part of the counter-attack. I don't want to stand still, whether they can see me, or not. If they can't see me, that movement provides an additional advantage, because they can't clearly tell where I've moved to.I get what you're saying and I do appreciate the idea but here are two possible problem's with the flanking idea:
1) I don't think the assailant is going to stand perfectly still in one spot. This is why I said it will not be as textbook as the video makes it out to be. You have to flank him while somehow leaving your arm in the right position to keep the light directly in his face. Just try doing this yourself with a training partner who is allowed to react naturally and you'll see that it's easier said than done.
2) It's an extra step or movement that you're better off skipping. If I have my light shining in a bad guy's eyes and have succeeded in taking away his vision I would rather take maximum advantage of that moment and drop him with one move (say a right cross to his jaw or a palm stick to his clavicle or sternum) instead of taking unnecessary chances with two moves (step to his left first while somehow keeping my adrenaline filled arm where it needs to be to keep the light perfectly in his eyes and then drop him).
Have a look at the second video again and look at the sequence where the female is at her car giving the bad guy the business. She has the light in her left hand directed toward his face and going to work on him with the weapon in her right hand. Which would have a better chance of saving her in a fast paced, adrenaline filled attack; that which she's doing already in the video or trying to flank him in between shots with the weapon in her right hand?
I'll reiterate that I most certainly appreciate the concept of flanking someone while shining a bright light in his eyes but I think that concept takes second seat to simply attacking him while you can as soon as the opportunity presents itself. James Williams himself does teach such a concept as evidenced in the video below.
Remember, he can't see what movements you're making behind that light anyway so deliberately looking to flank him is unnecessary imo. If you just so happen to flank him during the altercation then that's great. By all means take advantage of that moment if you do. But I would rather focus on surviving and escaping instead of fixing my mind on any one tactic, be it flanking or otherwise.