Yeah, thought maybe that’s what was going on. But I guess the next question is whether departure sequencing depends on the specific plane or just the flight number. If it’s the flight number then I would think they could swap frames and simply move the departure back up (but still late, just “less delayed”).

another possibility that ocurred to me is the inbound crew on the earlier flight might have been done for the day and the new crew for the 5:15 flight couldn’t start earlier to take the 4p flight, even with a ~30m delay.