Rarely used N length chain of words.

I.e. a high frequency 3-gram is "by the way".

A low frequency 3-gram is "elephants speak french".

But if you go too far the other direction, too many common n-grams, then it becomes platitude-speak, and nobody wants to read that either.

I don't think the paper really deals with that side of things. Just readability in terms of cognitive load.

correct, and that's largely why writing is a creative endeavour.