I appreciate that you or anyone you know can’t do it, however that doesn’t take away from what proper pistol squat form is. I’ve done literally hundreds of pistol squats without my knee passing my toes. The pic you showed has several form issues regarding a proper pistol squat (possibly knee too far forward though tough to tell from that angle, exaggerated rounded back, off leg should be straight, off foot shouldn’t touch the ground).
A pistol squat is an advanced technique that must be learned and progressed to. I would say that most can not perform a pistol squat let alone a proper one. It took me some time to perfect it and that was already coming from a significant squatting background and Olympic lifting (squat snatch and squat clean and jerk). Stress on the knee comes from improper form regardless of the squat type. However, it is much easier to have form issues doing pistols.