https://americanmind.org/memo/the-constitutional-authority-of-state-legislatures-to-choose-electors/
Because the US Constitution says, "Each State shall appoint, in such Manner as the Legislature thereof may direct..," it is indeed something the US Supreme Court can and should review. The review would have to be limited to determining if parties other than the respective Legislatures made decisions with respect to the conduct of an election that were either outside what was approved by the respective Legislature, or outright in violation of what it had authorized.
It is an important distinction that the Supreme Court could not (and should not) determine the propriety of the election procedures that were followed. The USCONS only gives them the authority to determine if the procedures followed what the Legislatures had approved in Art II Sec 1. Interpreting the USCONS is within the purview of the SCOTUS, so it is entirely appropriate they should look at this case.