The Chinese government can't arrest anyone here because the forum is under US jurisdiction. But it can try to restrict access to its citizens, as the Russian government has done by restricting its citizens' access to the forum.
China as the state can ask to transfer persons from other countries - USA, India, Russia etc. in criminal cases, it depends from international agreements. Is there a Criminal offense charge on the admin of the local board of China and if they already traced the identity of the admin, if there is none and you cannot provide us with proof then you're just speculating, you cannot talk about extradition based on speculation.
We have a lot of Chinese in our country and some of them are investors and promoters of Cryptocurrency and they are free to go back to China without any issue at all, we have a POGO here run by Chinese and they accept Cryptocurrency so what's really the issue here.