That's not quite the whole story, for which you'd have to read the book.
I don't know what Grind said, I don't see anyone named "Grind" in this thread. I don't even see anyone saying "slaves didn't have it as bad as we thought." Unless by "bad as we thought" means the media fiction that slaves were universally beaten by universally savage slaveholders. In that case, then yes, slaves didn't have it as bad as that.
Do you think slaves were universally beaten by universally bad slaveholders?
the point is, does it matter? they were SLAVES.