In US law, the key phrase from Northern Securities Co. v. United States, 193 U.S. 197 (1904, "the first railway monopoly case") was "in restraint of interstate commerce."
The trust was deemed by the Supreme Court to have been created for the purpose of ceasing active competition between the companies who joined it (there having been recent strife between them in the news, the trust being the negotiated solution, to back this purpose up).
So, very different. A similar case would be if AWS, Azure, and GCP decided to create a holding company that combined all their assets.
Worst case, they lean less on their own brokerage. Best case, they excel and can begin billing everyone else for providing the service. Win/win.