Depending on how the scene is done the reader might be able to follow by seeing which line comes first and picking up details of the speakers by the quirks of their dialogue. This would require that the writer of such a book be sufficiently talented that the quirks of the characters' respective dialogues would become immediately recognizable to the reader over time, and if they were that talented, they probably wouldn't need to read a book like How Not to Write a Novel in the first place.
Plus, well, it's a quip intended mostly sarcastically.
The co-authors are primarily editors by trade and have admitted that each example they give is "adapted" from an actual manuscript that they had a hand in rejecting. In some cases, the original version was even more of an incoherent mess than the version given here. Just let that sink in.