The clause means that the federal government is obligated to ensure the states provide their citizens with a republican form of government. So they have every right to rule a state law unconstitutional on that basis.
“The clause means that the federal government is obligated to ensure the states provide their citizens with a republican form of government.”
Yes, exactly. The clause obligates the federal government, not the states. So only the federal government can fail to live up to that obligation, and thereby violate the clause.