it states that while you have authority to change the rules, it ominously states that "your efforts won't help you if you have no group."
Pulling out this particular bit to think about.
I wonder if this was a response to some of the things the designers feared would happen, and so they tried to give a much more realistic portrayal of GM power and responsibility.
We've all heard stories of DMs who act crappy towards their players, changing rules that benefit them or creating rules that people don't really want to deal with, but forcing the players to deal anyways. And this always leads to bad feelings between the DM and the players.
So, I wonder if the intent with that line is to remind DMs that even if they think this hyper-realistic, true to history, armor vs weapon chart including quality of steel and rolling for hidden fracture is interesting, if the players find it a burden you won't get a lot of traction.
Sure, you can homebrew a monster. If you make an unkillable grudge monster because the players forgot to chip in pop money, nothing good is going to come of it.
Maybe it isn't the place of the book to say that, and I don't really know why the 4e book would feel the need to say that, perhaps the designers felt bad behavior was a little rampant in 3.X, but, I think the intent is to simply state: "You as the DM have a lot of power. Make sure to use it responsibly, because without a group, there is no game."