In a sense, this question is meant to mislead you. The question makes it sound as if China’s political unity should have made it stronger than Europe and allowed it to maintain its lead over that continent. However, this is exactly the opposite of Diamond’s argument in the Epilogue of this book. Diamond argues that China lost its lead because of its political unity, not in spite of it.
Economic theory tells us that companies become stronger when they have to compete. We see this in individuals as well. Companies and people that have to compete must become stronger. If they do not, they lose to those who have become stronger. Diamond is arguing that this applies to countries as well.
Diamond says that China was politically unified and therefore had no real rivals in its region. This meant it did not have to compete. Because it did not have to compete, it stopped getting stronger. By contrast, the countries of Western Europe had plenty of competition. Because they had to compete, they got stronger. In this way, Europe got stronger while China became complacent and did not get stronger. Eventually, Europe overcame China. This happened because of China’s unity and Europe’s lack of unity.