Fatigue sets in quickly (and sometimes almost instantly). In my case it was a young girl who was a fine swimmer (on the swim team) and had been playing in the pool all day on a hot and crowded July day. She got a little far away from the wall in an area between 5-6 feet, very much like the video, and suddenly found herself struggling to break the surface of the water.
It happens fast, and can happen to kids you would never suspect would find themselves in trouble.
I am unable to view OP's site at the moment so I'm not sure if you mean there was something on top of her.
Anyway, in open water "being able to swim" should include a "rescue/recovery float" ability. AKA, the ability to float indefinitely with little or no expended effort (ideally no effort at all, but some people have body builds which make light sculling necessary). If they can't do that, they aren't ready to swim in deep water.
That is the standard we used when I was a lifeguard and swim instructor (well, instructor assistant).
No, there was no flotation device blocking the surface in any of the videos.
On a slightly more positive note, this is a clip from BBCs Fantastic The Human Body. It talks about and shows the natural abilities of babies to swim at a very early age. If you're politically correct/sensitive, you may want to skip it. It does show genitals. https://m.youtube.com/watch?v=fbSCSHzXkrI