Not to infer too much into Kanye, but the statement works on a number of levels. I think he meant if you’re still talking about “400 years” all the time, at a certain point you’re making a choice to do so. But technically there was only 250 years of slavery, so if you’re saying 400 that means the other 150 were by choice.
That's how Rush explained it. Kanye was saying, y'all aren't slaves NOW unless you choose to be.