That means that we should get rid of monsters, theft, murder, assault, fraud, evil leaders, etc. from the game as well. If we're going to be making the game better than the real world, there's lots to remove from D&D.
Additionally, there really isn't a "cope with slavery" to be had. If you are okay with fantasy slavery because it's fantasy and doesn't touch the real world, play with it. If you are not okay with it for any reason, don't play with it. You aren't forced to cope with slavery.
That quote was replying to what Kunadam said; " if today's sensibilities (or should I say, US sensibilities) cannot cope with slavery in a fantasy setting, then the source of the slaves does not matter."
I don't expect anyone to cope with slavery. If someone doesn't want it in their fantasy setting, obviously, they won't have it there. And if you join a game with slavery and don't care for it, obviously, you will leave that game.
I'm not against mature settings or themes. I play VtM where everyone is a posthuman nihilist immortal who broods all the time and assaults people to survive, in a world circling the drain with very little hope of salvation. And that game has received it's own share of criticism and ridicule over the years (if anyone remembers that "Real Stories of the Highway Patrol" episode).
But D&D, to many people, is the definition of what an RPG is. Like it or not, it's the flagship. It's come under fire a lot over the years, and in the current environment, a lot of people (which includes would-be consumers) are touchy about certain topics. And mayhap they have a right to be, that's not my place to judge.
When marketing a game of "heroic fantasy" for players of 12 and up, I'm sure trying to make D&D seem "family friendly" is a high priority. If you're a young player, you want to be able to show your parents your books and say "see, it's all for fun".
It's a little hard to sell that with some settings. Ravenloft and Dark Sun, especially (though in truth, most settings have some vile stuff in them, like Zhentarim, Thay, or the Scarlet Brotherhood). Now us older gamers might see things differently; we're more mature, we want more mature settings.
But WotC likely fears that a book with a slipcover saying ADULTS ONLY won't help their brand. And while it's probably true that 12 year olds can handle Dark Sun, I sure as heck wouldn't want to be the one to make that judgement call. I'm sure WotC wants to do that even less.