First, a lot of them did get in a better situation. Not all of them stayed. And to those that did, they now at least sort of had dignity. And yes, everyone has to work for a living unless they're lucky. But they have the CHOICE to work for someone else. They have the CHOICE to go somewhere else. You can't tell me it doesn't make a difference.
Second, for those whose situation only marginally improved, that's because of the Jim Crow/racist laws that were put in place following the end of slavery. Black people went from having no rights whatsoever to some rights. Not as much rights as whites have, but some rights.
In other words, "no foreseeable income in a white world which collectively looks down on you" is very much tied together to a white world where blacks are enslaved to begin with. A white person being "nice" to their slave and a white person being nice to a black equal are completely different things.
I highly, I highly doubt the slaves were happy as they were. The odds of being caught while escaping was very real, which probably scared lots of them from trying to. Beyond this I highly doubt the slaves all of a sudden got treated even worse once they were freed, whether they decided to work for their former masters or otherwise. I'm sure the pay was barely enough to get by. I'm sure the hours were long. I highly doubt it was any worse than while they were actual slaves though, that's the thing. The only difference is now they are human beings with some amount of rights, whereas before they had none. They went from nothing at all to just a little bit.
And even with all the discrimination against blacks at the time, some of them would indeed go on to become rich, educated and much more highly successful than many poor whites would be of the same time. Yes, even with all these whites looking down on them. The odds were grim, yes. But how could you argue the odds actually got worse, than while they were slaves?
edit: the problem is that
a) his basis that "the slaves were actually treated fairly well" is a giant presumption already. Slaves were beaten, abused, raped, etc on an outright regular basis.
b) Ok, fine. Whatever. Let's just say while the poor whites in the north were working their ass off in factories getting their fingers chopped off, the black slaves got to just sit down and play nintendo all day while their sugar daddy master baked them a cake. They are still SLAVES, and moreso, they know it. I can be nice to a dog or cat but it's not equal to me. They were a 'plaything companion' at the very, very best. They know the only reason their master is "nice" is because their master doesn't take them seriously to begin with.