It might not have been successful, but query why Trump didn't argue Section 230 shields him from liability for reposts under Section 230?

It is settled law that Section 230, which is often thought to only shield intermediary platforms from liability based on the content posted by users, also protects individuals from liability for reposts of otherwise actionable content. The classic example is that one reposting a third-party defamatory claim is not themselves generally subject to liability for defamation.

Could Trump have claimed analogous protection? Probably not. One limit to Section 230's protection identified by caselaw is where the actor induces illegal content. It could be argued that Trump's act of posting (rather than the post itself) was illegal and, therefore, not protected. So it may not have helped him, but that an argument has little actual likelihood of success has never stopped him before. Well, chances are he'll have another chance to try it in the future. Stay (re)posted!