Jens:
Figure 6 shows the blade swinging in a large downward arc (arm pretty much fully extended) that would end either left of the knee, right of the knee, or in the knee. If it arcs left of the knee it could sever the femoral artery or other important parts of the anatomy. If the arc goes outside the knee it would likely throw the wielder way off balance as others have noted, and leave him exposed to almost any kind of counterattack. The author seems to assume that the target of the strike will stop or slow down the blade, as shown in the "impact lines" in the part of the diagram labeled 3. As most of the people in this forum know, even a tree branch doesn't always stop the swing of a khukuri, let alone the limb of a human opponent. And what if the opponent moves aside and the blade has nothing but air to stop its swing?
I don't claim any special expertise on the subject, but it's pretty clear that khukuris tend to be a lot heavier and more forward balanced than most fighting knives, and therefore many of the techniques would be different, both for effectiveness and safety. Especially techniques that require precise control of the blade and changes of direction.
Also, the picture shows a man standing virtually upright. It's not clear if his right knee is bent at all, or how much. That has a direct bearing on whether the swing of a heavy blade would throw him off balance toward the right. Martial arts in general place a good deal of emphasis on footwork and balance. Maybe the text goes into that, but Figure 6 by itself certainly doesn't.
I've spent a lot of time looking at different kinds of knives on the Bladeforums Exchange, even though I rarely buy them. I've seen hundreds of blades, and I pay attention to length, weight and other features. Interestingly, most people selling knives don't even give the weight, which suggests that they don't consider it an important factor. That might be true for relatively lightweight knives. The vast majority of blades that people consider "fighting knives" (sometimes by a stretch of the imagination) are smaller, lighter and balanced differently than most khukuris, including the military model khukuris issued in Nepal and to troops in the world wars.
In summary, I would take McLemore with a grain of salt, at least where khukuris are concerned.