Not all services in Jami require TURN, but as I said, Most Chinese users are behind NAT.
This post and official blog both claim that two users behind NAT need to communicate through TURN.
As much as we like to say that Jami is completely without server, sometimes it is not exactly right as we have seen. However, TURN is the perfect compromise for situations where a fully peer-to-peer connection is not possible
And terrytw also raised the issue.
In my own test, if the two clients behind NAT are set to no TURN, they cannot communicate. However, maybe my test is wrong. Hope to be verified by others.