Asisreo
Patron Badass
This reads, to me, as the same type of argument one would hear about how they wished the DMG had rules for longer resting times or a way to adjudicate social encounters with better structure: it simply shows you haven't fully read the book.I think the hobby would be better served with practical advice about table management, how to adapt adventures, customizing campaign settings, tactical suggestions for encounters, as a small guide for beginning DMs - and not a tome given equal importance to the PHB.
Table Management is discussed roughly from pages 6 and 34-36. Some sample advice I can pull is: Choose a playstyle based on your player's tastes and your skills as a DM and Including handouts to players concerning mechanical restrictions, campaign backstory, and their immediate starting area.
Customizing Campaign Settings is practically the entire book, but much more focused on the entire Chapter 1.
Building Interesting Encounters is discussed in the encounter building section just before all the colorful tables people get distracted by. Example advice from the book would be: adding a variety of terrain and elevation, features that facilitate movement for optimal play, and a diverse set of monsters. Y'know, the advice that gets millions of rounds of applause on the internet as if it was something the OP used their genius intellect for when its just something WOTC told you as advice from the beginning.
As for a monster's tactical abilities, that's simply discussed in the MM right next to the monster since tactics vary from monster to monster. If you want a very tactical monster play, you may want to look at TheMonstersKnow.com to know what a monster would do if they were perfectly tactical. I think having monsters act in ways sometimes suboptimally does help raise both versilimitude and diversity in strategy that lets a player take advantage of glaring weaknesses for huge effect. Sure, a Devil might kite the players and play a safe victory but they might also enjoy hearing the sound of fragile elf bones being broken so they might approach despite giving up its advantageous location.