Your reading comprehension has failed you again. Were any of those others that stayed afloat carrying an extremely heavy air tank on their back? No of course they weren’t. It’s the tank that means you can’t stay afloat without a vest BZ - of course a swimmer can, but not a swimmer carrying the weight of a tank without flippers.
I used to wear one of these Draeger rebreathers as a frogman. They are neutrally buoyant; the breathing bag balances the rest of the rig's weight. The little oxygen bottle can last about 3 hours under the right conditions. To become more buoyant, you just breathe in more oxygen from the bottle and exhale it into the breathing bag.
Now, I don't disagree with you in essence, that is, I'm not seeing a diver in the pictures. But you are wrong that a diver would necessarily have a heavy bottle on his back. A diver with a closed-circuit rebreather can easily maintain any depth from 30' down, to on the surface with his head exposed, simply by breathing more or less oxygen from the bottle into the breathing bag.
This is the chest-side of a Draeger, showing the breathing bag.
