Good luck getting a libel claim out of what Grendel said. His statement was offensive and false but I do think think a reasonable person could consider it anything other than reckless rhetoric only used to be provocative and inflammatory.

Being offensive and (knowingly or negligently) false is sufficient for libel against a non-celebrity. Only for celebrities (including public officials, etc.) is it necessary to prove that it wasn't just rhetoric. But I think this fits under "defamation per se" which makes it always count anyway.

Section 230 only applies immunity within civil law. In some states, libel can be criminal; harassment can be criminal even at the federal level. I am not a lawyer, but I know that investigating this issue could be costly.

Additionally, the offensive user is certainly still liable under civil libel law, which may give Hubski the obligation to reveal his identity during discovery. Is violating anonymity something that Hubski is prepared to do? (Granted, this may still apply even if Hubski does act to take it down, but it is less likely)

Remember that even if a lawsuit is ultimately shown to be unfounded, it can still have significant up-front costs. (This is less true in countries other than the United States, but most other countries do make ISPs clearly responsible for libel that they don't remove once they are informed).

Finally - absolute domination of free speech is not an ideal goal. Humans are irrational, especially in social circumstances, so the "it is always better to know" theory does not apply. One negative instance can drown out thousands of positive instances; this sort of accusation, even unfounded, can severely effect the ability to apply for a job.

If Hubski wants to become a center for hatred and harassment, then by all means leave it accessible. If it wants to become a center for free speech, a clear and enforced policy for "this kind of speech is not protected" is absolutely necessary.