What is true is that the troll's message will not appear in the list's archive. (That's how I became aware that one person was actually a troll; I went to the archive, and, huh? This guy is not there! And neither are my replies to that guy. What the .... Then it immediately dawned on me!)

But, yes, trolls will be replied to. Because, remember, they are not even going through the mailing list robot. They are just sending mail. Of course the troll's mails can be replied to and go back to that person, and to everyone on the CC list that he or she put in.

Not only that, but the troll can include the list address on the CC: line! A reply to the troll will include quoted material from the troll (typically), and since the person responding is a valid subscriber who is allowed to post, that quoted material gets to the list.

So all the list subscribers end up seeing:

  On Monday, October 6, 2014 T. Roll wrote:
  > Inflammatory crap  ...

  I disagree with your inflammatory crap!

Nobody on the has the original message with the full inflammatory crap (except those on the CC: list of that troll thread including the person writing the above response). But thanks to this reply and others like it, everyone on the list continues to have glimpses into what T. Roll thinks. (They are is even trimmed nicely to give the list members just the most inflammatory parts!)