tl;dr They are reasoning about why the cognitive effort during dialog seems lower than listening or providing a monologue. They conclude that we share the load of aligning our representations of things and reusing the work we've done while talking.
We conclude that humans are ‘designed’ for dialogue rather than monologue.
This fits my experience. I've always sworn that dialog podcasts are more enjoyable than monologue ones, so I was hoping this was about that, but it's about first hand dialog. I wonder what the authors would say about third party dialog being easier.