The top pulley is easily visible, although it would have to be removed to see how much crud has collected inside the hub. If you can't see the upper pulley in the photo, you are blind. Besides, the time to clean and inspect the complete rear derailleur would have been while the wheel and chain were off the bike being replaced. Obviously that was not done.

There's more to a proper replacement procedure than just throwing new parts on. Cleaning and inspecting the associated parts when replacing damaged, dirty or worn-out parts is not OCD, it's good, competent mechanical procedure.