Heard a high school coach tell team to swim flat?

I was swimming laps yesterday and I overheard the swim coach tell the high school swim team that they should be swimming flat with no rotation of the hips. He told them that at no point they should not be kicking on their sides and that swimming flat decreases drag. I've had one other person tell me the same thing when I first started swimming and I ended up with severe shoulder (rotator cuff) pain. It wasn't until I learned how to rotate from side to side that the pain resolved.