Delta allows their elites an extra 20 lbs on domestic flights. Your 58 lb bag would cost a non-status pax $90 on Delta.

I think a better way for the agent to have put it would be to say: "Since our baggage fees are among the lowest in the industry, we are especially strict about enforcing them."

For what it's worth, that $50 that Alaska charges includes up to 100 lbs. Even with your Diamond status, Delta would hit you for $175 if your bag weighed 71 lbs.