I can't speak to whom AD&D was aimed at. Who was expected to play it, and who was considered not the target audience. I'll defer to those who are in a better position to know.
As for the DMG, however, there's a lot of evidence to suggest that Gygax & others very much desired for the game to have "secret rules", and to cement the Dungeon Master's place as the final authority on all things. Gary's tone is sometimes very unfriendly when it comes to the players of the game, and at times comes across as antagonistic towards them. Those who played at his games, however, paint a different picture of the man. The author of the DMG seems to be a rules-obsessed autocrat, the kind of person who would make an ultra-lethal dungeon out of sheer spite (see
Tomb of Horrors), but the people who knew him describe a man who had a much more laissez-faire approach to the rules of the game, and seemingly wanted everyone to have fun.
I can't explain the dichotomy, after all, there is this infamous quote in the DMG:
Here we have a quote that would seem to be made by someone who feels that daring to learn the secrets of the DMG is obviously a person who seeks to gain an unfair advantage in the game, or even cheat!
Why is the DMG advocating for hamstringing Wizard spell choices, to the point that NPC's will refuse to share spells for anything resembling a fair deal? Why is it keeping an important class feature of the Assassin not only secret, but even if a player asks the DM if they can acquire it, advises you to make them spend time and money in research that will automatically fail if you're not the correct level?
Or why the DMG makes it very clear that allowing someone to play a monster character is a bad idea- an explanation that takes six whole paragraphs that one could boil down to "I think humans should be the focus of D&D because it makes the most sense to me"?
I don't want to uncharitable- I don't know what was going on in Gary's mind at the time. Maybe he'd had to deal with an abusive player, and wanted to give prospective DM's the benefit of his wisdom. Maybe it was a joke that didn't quite land. I didn't know him, and I can't ask him.
I will say that I know people who learned the wrong lessons from the AD&D Dungeon Master's Guide, and to this day, run very antagonistic games of D&D where player characters have very little agency or hope of success. Is that the book's fault? It's hard to say. But having had these sorts of individuals justify themselves by quoting the book directly, I think something went awry.
The thing that bothers me the most about the AD&D DMG, a book I value and treasure, is that it really doesn't discuss how one becomes a Dungeon Master.
What is the process by which this occurs? If players can't read the book, or even look at it, without deserving a "less than honorable death", where does one begin? And as wonderful a resource as the DMG is, it really doesn't teach one how to DM. It can help someone be a better DM, to be sure, but it's written with the idea that the read knows the basics already.
And perhaps that's the point, after all, it is called "Advanced Dungeons & Dragons". Maybe you were meant to learn by playing D&D? Or maybe nobody was that forward-thinking at the time?
It's one of the most frustrating things about the hobby that this difficult and vital task, of being the Dungeon Master, has often had a very shoddy on-ramp. I know I got started because I had a story I wanted to tell, and to this day, I'm still learning new lessons about it.
But I digress. "Is D&D for everyone?" is a question for the ages. Different people think different things about that, and perhaps, in the 70's, it wasn't intended for everyone. Thankfully, regardless of whether it was or wasn't, many people learned to play and love the game.
