The version of freedom of speech that is encoded in US consitutional law (the first amendment and the long history of supreme court judgements on the subject) is an example. There are forms of speech that are not protected and there is a mainstream interpretation of freedom of speech that does not see restrictions against defamation, incitement of violence, etc., as infringements of it. A "free speech absolutist" would see these as infringements of freedom of speech, so the term is useful to distinguish that particular view.
You're seeing the term used a lot on Hacker News because there are people here who routinely argue that any speech should be allowed on a platform that claims to support freedom of speech. These people are free speech absolutists (at least in the context of the debate about online platforms) and, if they feel their views on the matter are correct, shouldn't be offended to be called that.