I think the argument is that commercial recipes in the US are written in proportional notation, e.g. 1:2:3 sourdough, but recipes in countries which use metric give units, e.g. 1kg:2L:3kg. I also note that if you add small proportions of an ingredient, e.g. salt, it might be easier to change units in metric (5g salt) while it would be easier to write proportionally in imperial (0.005 parts salt) if you were then going to scale to to a tonne/ton of dough.

I have no idea if this is true but it sounds like a coherent argument that isn't just volumetric vs mass units.