I wondered about that—they stated that they processed videos to pull out keyframes. It makes me wonder how much that will or won't help with in-betweening.
Keyframes in digital video means a different thing from keyframes in animation; they could be related but usually wouldn't be. You can hide animation keyframes with a number of techniques, for instance by having different "keyframes" for different parts of a scene so the video doesn't show them all at the same instant.
(Rough generalization, but anime is more likely to do this with foreground vs background elements, while Western animation would do this with different characters. In anime the keyframe artists tend to draw every character in the frame, and put more of a personal touch on them, so it's easier to see their individual styles.)
Since this is a "sakuga" dataset, Mitsuo Iso is one of the most famous examples of really natural looking movement here.
When they're bad at it you get this effect where characters constantly seem to "settle" into a keyframe pose that looks realistic but too static, and then immediately go back into inbetweening that moves a lot but isn't physically possible. I feel like B-grade Disney stuff is the worst here but don't have an example on hand.
