“They don’t determine federal elections rules”
The appointment of the President by State Legislatures is not a federal election.
Article I gives Congress the power to make rules for the election of Representatives (and Senators since 1916), but no part of the Federal government (including the Supreme Court, all you Bush v. Gore fans) has ANY legitimate role in the appointment of the President by the Electoral College.
The Founders debated this for a long time. They discussed a role for the Executive, for Congress, for the Senate alone, and eventually decided NO role for the national government, unless the States could not form a majority to make the appointment.
Bush v Gore was a legitimate SCOTUS decision reeling in a rogue state supreme court -- as SCOTUS has done in the past -- and not unironically will do so again in Colorado, or do you support this Colorado decision?