Elbow flexion, on its own, has little to do with the upper back. For example, if the bar was light enough at the start of the deadlift, you could be leaned over and perform a type of reverse curl. That is elbow flexion without anything happening at the shoulder. Let me put the question back to you. What are the shoulders doing in the deadlift vs. the row? What are the scapulae doing? What muscles are affected? What are the actions of those muscles?