Better phrasing would have been "the important tokens are roughly evenly distributed throughout the text", that was the intended reading.
For a textbook at least, it often seems to be the case that you need to have fully ingested the big picture ideas of one chapter to move on to some later ones, but this seems to me at least more like updating your model, rather than sampling context from the whole book (I mean it is an analogy of course, so neither matches perfectly).