He has a right to sign a written confession. His confession will be introduced in court, and he might well be convicted based strictly on his confession. There is a reason no defense attorney has ever tried to get a confession thrown out based on the First Amendment, it's not a First Amendment issue.Really doesn't matter if he wrote those or not. Free speech is protected by the First Amendment in this country. Still does not keep the guy from protecting himself from some half-wit sticking a gun in his face. That guy could not have known what was put on social media and should have expected someone to blow him away for jamming a gun in his face.
Did he claim he didn't post the statements himself? If so, his attorney might convince the jury they were not his statements, or at least that there was reasonable doubt he made them. But that would be arguments made about statements that were already in evidence and would be considered by the jury.
No one is arguing he has no right to defend himself. We are just saying the First Amendment does not preclude his public statements from evidence.