his first amendment rights allows him to say so.
The indictment specifically says he had the free speech right to lie and he also had the free speech right to say he won. But what he wasn't allowed to do is to TELL LIES which caused him to ACT in an illegal way and knowingly pressure officials to lie for him. It's not about free speech, it's about the ACTIONS and BEHAVIOR he took afterwards, and that's what he's being indicted for.
What he didn't have the right to do was:
1. COUNT ONE: Title 18 USC 371: Conspiracy to Defraud the United States.
this count covers everything Trump did in his attempt to overturn the election: the fraudulent elector scheme, bullying Mike Pence to reject the certification, and sending that crowd, with their improvised weapons and that noose, to overrun the Capitol.
COUNT TWO: 1512(k) Obstructing an Official Proceeding: By conspiring to obstruct the electoral count on January 6.
this count covers the conspiracy to send that armed mob to the Capitol.
COUNT THREE: 1512(c)(2): Obstructing an official proceeding
this count covers the actual disruption of the electoral certification.
COUNT FOUR: 18 USC 241: Conspiracy against rights
this count covers the conspiracy to deny American citizens the right to have their votes counted.