You said that both things ('importing new people and replacement breeding') contribute to overpopulation. Importing new people can only contribute to overpopulation if you consider it locally. If you consider it globally moving X people from A to B cannot possibly directly affect the total amount of people in A and B. I considered overpopulation only in Europe and not globally because neither did you.
You're not understanding. Overpopulation involves the worldwide figure. The point of the statement is that immigration increases overpopulation by moving people to a new place, and then replacing them in the original place.
