That makes no sense to me. That has nothing to do with what offset is needed, only the axle width does.
If the tire fits, make the axle wider so the wheel fits.
I can kind of see the logic with a Duster and a stock axle though. The hips are wider to a degree than something like a Dart, and the axle width is fairly narrow. So fitting a wider wheel without changing the axle width would push the tire out and make the wheel deeper.
But if someone is working hard to build a sleeper, they can use a wider axle and more backspacing to make it look right. The inner wheel tub really has no bearing beyond what size tire fits.