I specifically asked what the underlying cause of this problem was, and he said he'd been told that Dell had decided not to share that information, but that he would be working on getting some sort of official Dell Support article published about this issue. He did so and then replied back to me saying that he had found internal documentation indicating that all of the following systems had this 1080p limitation on their HDMI outputs: I then reached out to an official Dell Support rep who has been helpful getting issues like this escalated internally.
I saw threads reporting this issue involving the Inspiron 34, both with multiple users confirming this behavior.