> Given the weird nature of this case, and how only one viewpoint about it was allowed on social media, I wonder if that constitutes defamation on the part of the social media companies, since they were in practice making an editorial decision that he was bad.

You yourself acknowledge that it is a "viewpoint", or the term more common in law an "opinion". Opinions are protected speech by the first amendment (not section 230) and as such Facebook cannot be liable in any way for promoting one or silencing another.

To be liable for defamation, you would need to be stating false facts with reckless disregard for the truth (or malice, if Rittenhouse counts as a public figure), not just promoting an opinion. This is regardless of section 230 (which might or might not provide them further protection).