A cup is always by volume, so you can’t really criticize it based on that. A metric cup is 250mL, imperial is ~230mL it’s nowhere near as “shitty” as “a bag”, it’s just a unit of measure that you are unfamiliar with. It’s used quite a lot in baking.
I would have guess two pounds, based on how often I see that in the grocery store. Though five pound bags seem to be getting more popular over time.