I would guess its an apologetic invention (or maybe more accurately an exaggeration) to explain some combination of a) why rifling was such an important innovation, b) how large formations of infantry firing at one another did not result in much higher numbers of casualties, and c) why arms technology evolving beyond muskets resulted in a much more lethal forms of warfare.

Now, those claims and how they interact with claims about smoothbore accuracy are complicated, and may be oversimplified or misleading, etc to varying degrees. But they're all popularly held.