Rear Differential Fluid Change - Turbo

Dealer told me I needed to change the rear diff fluid at 30k miles (I get free oil changes for 3 years otherwise I would never take my vehicle to the dealer). They said the rear diff fluid was dark and dirty. I was skeptical. I bought the gear oil from the parts department and did the oil change at 34k miles myself. They wanted $280. Other than some gunk on the drain plug magnet the gear oil looked fine to me. Was I lied to? (2022 Mazda CX-5 Turbo)