I don’t think many people use VPN at all. You only need it to bypass the firewall to browse certain foreign websites. Only a small fraction of people does that.

But it is true that it would not be very effective against determined trolls posting from, say, Taiwan, because they can simply use VPN servers located inside China. Without showing the IP location, people will just endlessly accuse one another for being Taiwanese trolls without any proof. Surprisingly the flame wars did tone down substantially after this was introduced.