Do you
really think any amount of text in a book could convince a jackass DM to not be a jackass? Because yes, the DMG tells the DM that they're in charge not the rules, which is a good idea. It let's people know that house rules and tweaks are part of playing D&D. But let's look at the entire paragraph instead of just taking things out of context
What’s the right way to run a campaign? That depends on your play style and the motivations of your players. Consider your players’ tastes, your strengths as a DM, table rules (discussed in part 3), and the type of game you want to run. Describe to the players how you envision the game experience and let them give you input. The game is theirs, too. Lay that groundwork early, so your players can make informed choices and help you maintain the type of game you want to run.
If someone is going to ignore that last sentence along with several other sections in the DMG I'm not going to bother looking up, no amount of text is going to make them a good DM. There are obviously other ways of running games. They can
never guarantee that one person at the table, whether GM or player, won't ruing the experience for others.