I don't know that the original question of "is this a mistake" has been addressed fully, by any of the above. Certainly the inorganic Chemistry section incorporates mass balance and has some energy balance concepts as well, likewise with fuels and combustion. But, like you, on first blush I would consider the bulk of the conservation of energy principles to lie in a different section like perhaps chapter 22, based solely on the chapter title.
In really these concepts are sprinkled all through the Fluids and Thermodynamics sections of the text (Chapters 14-25). So, no, it's not a mistake for PPI to point to 20 and 21 and say "study energy and mass balance in these chapters". It would simply appear they want you to focus on the mass and energy balance aspects of the materials presented in the chemistry and combustion sections for this area of test prep (those are key concepts in those chapters I can assure you). As has been noted above, there are likely to be problems on your exam that stem from these sections, because historically there have been, as was the case in my own exam, as well as in the exams NCEES has published for use in self study, and the point PPI is driving home here: the questions from these chapters will deal with energy and mass balance concepts.
As for your second question: which chapters should I study for energy and mass balance... well, I sorta already touched on that. The fluids and thermal sections of the text all incorporate these concepts quite heavily, however Chapter 22 has probably some of the more intensive focus on energy balance. Just keep in mind that any time you are dealing with a system that is dynamic in any way both mass and energy balance is going to be key to proper understanding and breakdown of the problem into its basic elements. When you have a static system, the key concept is going to be balance of forces. Hope this helps.
