I don’t think the logic follows in that way.
Trump is making Twitter into a public square by using it for communication from the president of the us. Twitter is not the public square without this public (governmental) use.
In a similar way just because a newspaper reports on a government news story doesn’t mean that they are required by the first amendment to publish your letter to the editor.
RE: Judge rules Trump can’t block people, what does this mean for twitter going forward?