I can attest to this method. Similarly to the GP, at some point in my early teens while swimming at a small lake, I decided to try swimming across the lake (more of a cove) and back. I was never a great swimmer but I was confident I could make it because I knew I could just flip over and leisurely kick if I got tired. Sure enough, my arms started to burn about halfway across, so I just flipped over and eased the rest of the way.

It's not nearly as fast as "proper" swimming but I'm sure I could keep it up approximately as long as I could walk. Maybe even longer. It makes for a great option in dangerous situations, and is likely a good fundamental technique to teach anyone just learning.