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. Yeah they can and they will be refused as that forum is not against USA law.
Now if that forum section constantly promotes illegal shit like child sex which is not legal in USA 🇺🇸 or China 🇨🇳 the us may go after the person running the section.
Some shit is not legal anywhere and the USA 🇺🇸 would likely say here is this pedophile that was promoting child sex in china 🇨🇳 and BTW you have a person in china 🇨🇳 doing something bad lets swap them.