It's the same principle as Pass Rushing. You want to punch through just over the shoulder to immediately get onto the back of the defender. IF (because in looking back quickly I didn't see anyone too horrible) there is somebody way too high, it could be because this group was a mixture of college players, HS players and even some guys doing this on the first day. The point was to see a whole mixture working through to see how the drills benefit most experience levels. Two kids I specifically remember going through these day 1. But you can only swim as low as the defender and the Coach in the video is standing up. It's possible his posture forces a slightly higher swim. But most of these are foot work focused, especially for the new comers. Personally, and everyone might be different, I feel until the feet are right the upper body is a waste of time.
I now see what you are talking about and it is what I had previously said. The players who were at a certain level are golden and the ones who are just being taught weren't being corrected yet since the focus for them was footwork. if you go to 5:10 these next two players are the players who I had worked with the most/longest. You can see how they don't swim high what so ever. They low flip their back arm over the catch arm with a low punch. This is what we would teach once the footwork is right. Glad you asked.
Watch TJ at 5:35 not use any height on his swim at all. This is ideal. But he was several years into doing most of these drills at the time so was expected of him.
