These guys are riding in an extremely rural area of southern Guangdong China. You notice there is almost no one around? All the young and middle aged people are gone to find work elsewhere. If they wish to compare Kumamoto Castle in Japan to a remote and run down small village in China, that is not a fair comparo. Large tourist sites, such as the Forbidden City, are very well maintained in Beijing and the larger cities.
Public spaces in China are not maintained by home owners but by the government, therefore they can be really run down. Why do the government's work?
Much of China is still rural. They use mud for walls, and they do not last very long. If no one is living there there is no need to maintain it. When someone with money moves in then it gets maintained.
I'd be very careful riding around various parts of China, because much of China can be closed to foreigners. The police will confiscate your bikes, fine you and them deport you. Worse still if you meet a local militia, you might get beaten up. Maybe they know the area well enough. Their Mandarin is fluent enough.