The answer to this is found in the Epilogue. Basically, Diamond argues that China’s connectedness ensured that it would not have to compete with other countries in its region. This ended up being a disadvantage because it allowed China to become complacent rather than pushing ahead and becoming more and more technologically sophisticated.
Diamond argues that China’s geography allowed it to be easily united. While it did have a north-south axis, it was not hard to move along that axis. There were two major east-west rivers, both navigable, one in the north and one in the south. The land between them was easy enough terrain to allow a canal to be dug between them. In short, China did not have geographical obstacles to unification. It was geographically connected and that allowed it to be politically connected.
Because it was politically connected, it came to be at a disadvantage compared to Europe. For much of its history, China never had to compete with any serious powers in its own region. This meant that it did not have to make technological progress. It could “stand pat” and still be dominant. This was fine until the Europeans came. The technology that they had created because they had to compete with one another was far beyond what the Chinese had.
Thus, connectedness became a disadvantage.