Sorry if my wording led to some confusion, but DL and NW don't currently have reciprocal UGs, so what you've been told is correct. By 'premium seating', I was referring to Y seats that Elites can choose for free but are not available to non-Elites (e.g., exit rows, first several rows of Y). Depending on the airline, this is typically called 'premium' or 'preferred' seating.