My understanding is that new cases of the virus in the coastal cities of China are quite low, and that they have a heavy handed testing/monitoring system in place with personal accountability given to the party reps stationed in each residence complex.
Also, Japan-China will always have trade and geopolitical considerations, for better or for worse. Lots of Japanese companies have offices and manufacturing plants in China that they'd like to continue operating as well as they can.
They’ve shut off zhejiang, which is the richest coastal province and the heavy weight of trade...not to mention it abuts shanghai. I’m guessing that travel between japan and China is effectively disabled along with the rest of the world, given that even if a Chinese wanted to visit japan, they are looking at a 2 week quarantine on the way back, perhaps also on the way there.
You're right about 2 week quarantine when a traveler gets back to China. Also I believe inter-province travel is quite restricted right now adding to said difficulty of travel.
Also, Japan-China will always have trade and geopolitical considerations, for better or for worse. Lots of Japanese companies have offices and manufacturing plants in China that they'd like to continue operating as well as they can.