He's saying that if borders at the time had been more open to immigration from Europe, the European workers could have displaced black workers who migrated inte

He's saying that if borders at the time had been more open to immigration from Europe, the European workers could have displaced black workers who migrated internally (within the US that is). I should not think he ever supported 'letting Detroit factory owners hire only whites', but rather he's saying that his previous position that immigration (from abroad) has little or no disadvantages to domestic workers did not take into account this example and he has adjusted his position accordingly. That an economist, even one of great stature such as himself, would have been blind to endemic racism within the power structures of society is, well, the point of his article.