The quoted comment is not correct.
The First Amendment makes defamation cases very difficult to win.
A case of "defamation per se" is a bit easier than a standard defamation case, as the plaintiff does not need to prove damages, but she still otherwise prove all the elements of her case, which are:
a false statement;
about the plaintiff;
published to a third party;
causing damages to the plaintiff.
And if the plaintiff is a public figure or if the defamatory statement addressed a matter of public concern, there are all sorts of procedural hurdles that will make it very difficult for the plaintiff to overcome the defendant's First Amendment rights to speak about her or her conduct.
In this case, because the moderator in question is a volunteer, this isn't a case of defamation per se, so she would need to prove damages, which I suspect would be minimal, especially given the lopsided reaction to the post in her favor.
But the most problematic piece will be the second prong, as the SE statement about the case never identified the moderator in question. One of the posts did, but only because Monica herself edited it to include her name.
Proving that the statement is false will also be difficult, as the courts will require not just that the statement is literally false, but that in the overall context of the entire statement, it left readers with an impression of her that was more damaging than the full truth.