You will also see plenty of cases where a screenshot captures incoherent frames.

Squash and stretch is a whole art style that relies on unrealistic frames.

You're thinking of smear frames. Squash and stretch are animation techniques that are perfectly coherent. Smear frames as well contribute to an overall coherent animation. They're a counterpoint to the general idea put forward in this article, but it's also rarely ever relevant to this type of animation.