I think he is referring to the shot, powder, and wad individually. Each alone is equal to the bore diameter.

IMHO, this is one of those old wives tales that just won't die. It is completely irrelevant with the advent of choke, and petaled shot cups. Even with cylinder bored muzzleloaders, most shooters find that thinner (i.e. lighter) wads produce better patterns, as opposed to stacking them up just to match the bore diameter.

I suppose it is a good place to start for experimentation though, making adjustments to produce the best results.

“I left long before daylight, alone but not lonely.”~Gordon Macquarrie