You missed the part where the video's frames per second has to be higher than the audio frequency.
For this particular film the rather characteristic look of digital video is appropriate, whereas most productions would rather shoot with much fancier gear. But you'll find plenty DSLRs working on film sets these days, and you will see more and more convergence as high-quality sensors become a commodity. Black Magic have a pocket camera that delivers 13 stops of dynamic range.
With two time samples you shouldn't be able to learn anything about the state of waves in a pool, but if each sample is a photograph with lots of pixels you can actually tell a lot.
> Because of a quirk in the design of most cameras’ sensors, the researchers were able to infer information about high-frequency vibrations even from video recorded at a standard 60 frames per second. While this audio reconstruction wasn’t as faithful as it was with the high-speed camera, it may still be good enough to identify the gender of a speaker in a room; the number of speakers; and even, given accurate enough information about the acoustic properties of speakers’ voices, their identities.