When someone repeats an argument that has been proven false /badly argued many times before, but keeps repeating it in hopes of drowning out opposition or derailing a thread. Yet not disruptive enough to get banned on forums, as it wraps itself in non-hostile, nicely written sentences.
How exactly do moderators deal with this kind of behaviour?
It would be a Gish gallop if they do some brigading; that’s how they usually scare off OP. Since he did not know he was making a controversial statement, and did not pre-emptively stop the Gish Gallop.