These chapters are basically an extended critique of the evils of chattel slavery and of the American policy against the natives. He does indeed say that
"As soon as it is admitted that the whites and the emancipated blacks are placed upon the same territory in the situation of two alien communities, it will readily be understood that there are but two alternatives for the future; the negroes and the whites must either wholly part or wholly mingle."
Why? Because blacks have been so oppressed and there are so many of them in the south, that it would be impossible for them to live together with their former oppressors after being liberated, both would fear and hate the other. And what does he mean by wholly mingle? He is advocating mixed race marriages, something that wasn't legal everywhere in the U.S. until the Supreme Court forced it to be in the 1960s.
There are surely lots of passages in the book that 'read' as racist by modern standards, but by 1830s U.S. standards, Tocqueville was very progressive.