If I understand correctly, he's proposing changing 230 from permitting service providers to moderate user content using their own judgment and values, to moderating content using the First Amendment as the baseline. So, removing a post with a controversial opinion is fine under the current law, but not covered by the proposal. The whole "publisher versus common carrier" argument is sidestepped.

My question is, what'll happen if somebody moderates beyond the bounds of the First Amendment? If Twitter deletes an unpopular opinion that's legal to publicly express, would the poster be able to sue for unlawful removal?