Any methodology for getting more information from less information is going to involve guess work at some point, surely? This guess work just happens to be helped along by having the results of (I assume) millions or more separate videos where they already had the higher speed footage.
This is also not extrapolation, rather quite the opposite, it's interpolation.
But that's what our brains do too. We might think we're seeing the world as it is, but much of our smooth and consistent view of the world has a lot to do with our brains patching things up to make it look that way.
The thing is, that it does raise some questions - I can see things like fake super-resolution and fake slow motion and other alternative realities generated by machine learning easily fooling humans. Once things become sufficiently advanced, who's to guarantee will we always know what the true source is? Could future training be done on data that itself was generated via machine learning. That brings up a load of other questions. Interesting times ahead...
Don't our brains also extrapolate? I am sure our eyes feed the brain at a limited frequency, and the brain combines and extrapolates the images to create the illusion of smooth streams.
How high is that frequency is yet unknown:
"Tests with Air force pilots have shown, that they could identify the plane on a flashed picture that was flashed only for 1/220th of a second.That is identifying. So it's pretty safe to say, that recognizing, that SOME light was there is possible with 1/300th of a second." http://www.100fps.com/how_many_frames_can_humans_see.htm
This is also not extrapolation, rather quite the opposite, it's interpolation.