Not sure what this actually means - liable for what?

They already ban tweets required by law. What other liability is there?

If Twitter were held liable for user's posts they would probably ban all right-wing content since that's were most of the libel I see comes from. For example reddit tried hard to keep /r/TheDonald around but it turned out to be an impossible task. Especially in online right-wing youth culture there's a strong anything goes, ends justify the means, trolling for the lulz, strain where they find it more acceptable to spread deliberate misinformation.