This seems to confuse U.S. anti-trust laws with that of free speech. The two are separate and distinct. Big tech may have their share of anti-trust problems, but even breaking up the companies would not cure the speech "problem" you propose.
No matter whether they are big or small, private companies can make their own decisions about who to allow on their platform. The First Amendment only protects against most government censorship, not that of private entities.
Even looking to the "tyranny" over speech here, I struggle to see it. These companies have put guidelines and terms of use in place. For years, both the President and thousands of other users continued to violate those terms and were given a pass. They have ample evidence to show their guidelines were violated and therefore are fully justified in banning these actors from inciting further violence.