That is correct if the downstream panel is fed from a "Lighting and Appliance Branch-Circuit Panelboard".
If the downstream panel is a "Lighting and Appliance Branch-Circuit Panelboard" and is fed from the Main Disconnect (as in Service) it can use 310.15(B)(6) in that the feeder would meet the definition of a Main Power Feeder.
Well, since these feeder conductors are "to a dwelling unit" it would seem as though by the wording itself it is referring to a Main Power Feeder, not a sub-fed downstream panel.
If the paragraph used wording such as "to a pump" it would it would cover more than just a Main Power Feeder.
Roger