If they're "poor", don't they have to rent or buy low income housing, and so "concentrate into economic ghettos"?

There is no policy concentrating the poor with each other, any more than there is a policy that concentrates the rich with each other. The market makes available different products, and different economic market segments purchase different products. With a policy, or without a policy, the poor will live with the poor.

And form "economic ghettos".