Well, I was an olympic-trials level 400 freestyler (and almost in the 1500) and can tell you that a long course 400m freestyle requires a greater level of fitness than a short course 400m freestyle. Or putting it another way, you can maintain a greater level of relative effort on top of the water while swimming the short course version of the event.
My argument addresses the OPs question of whether a flip turn and simple push-off is faster (either by itself, or by allowing for more rest) than swimming through it would take in open water, which isn't the same thing as considering dolphin kicking 15m off every wall, which would eventually lead to oxygen debt in longer events. In a typical distance event, after pushing off, you get 5m where you expend virtually no energy in a streamline position for only a few seconds, which allows for greater recovery (and is faster) than swimming that same distance on top of the water.
Strava
My argument addresses the OPs question of whether a flip turn and simple push-off is faster (either by itself, or by allowing for more rest) than swimming through it would take in open water, which isn't the same thing as considering dolphin kicking 15m off every wall, which would eventually lead to oxygen debt in longer events. In a typical distance event, after pushing off, you get 5m where you expend virtually no energy in a streamline position for only a few seconds, which allows for greater recovery (and is faster) than swimming that same distance on top of the water.
Strava