Good high-carbon steel is harder to make than mild steel. It used to be a lot harder to make; carburization in ambient atmosphere takes an enormous amount of time and fuel, and even then is hit-or-miss.
Because that's so, and also because the brittleness that makes high-carbon steel great for knife edges also makes for a very fragile blade if that's all you use, good knives historically have been made by forge-welding a strip of high-carbon steel to a larger piece of mild steel. The mild steel makes up the bulk of the blade, and the high-carbon strip is shaped and ground to form the edge.
You can sharpen that edge by removing metal, but if you do that enough, you'll remove all the high-carbon steel and end up sharpening the mild stuff. That can't take as good an edge or hold it nearly as long, so you end up with a knife that's dull no matter what you do.
This is also why sharpening steels are a thing, because good knives are usually still made this way. Stropping on a steel cleans up the edge profile without removing metal, yielding a sharper cut without shortening the life of the tool.
Granted, all of this discussion of the mechanics misses the point of the koan, which is all about moderation and humility, and nothing really to do with cleverness beyond the always wise counsel not to let it run away with you. But then, too, moderate discussion of the mechanics may in this case cut toward that end, so I don't really feel this an indulgence to excess.
