It appears the court has (correctly) avoided making a determination whether or not the respective state legislative bodies can select/control electors prior to a presidential election. Where it gets interesting is the circuit court has opined that state legislators cannot control/direct electors' votes after a presidential election.
This I believe is in error and will be ultimately overturned. Their logic is similar to Trump not being exonerated; that is, referencing a negative. Specifically, by noting that the constitution doesn't specifically state what a state can/cannot do, the judge has determined a state cannot.
A proper reading, with respect to the specific clause regarding electors, and of course the 10th amendment, is that states can not only select electors of their choosing, but can task them with casting votes in accordance with guidelines however the state wishes (as long as it is in accordance with republican guarantees).
Prediction: inter-state popular vote accord ruled unconstitutional, individual state elector control/direction by state legislative bodies upheld.
A distinction without a difference is how I would characterize their point.
There are several ways the “compact” conflicts with the Constitution.
But I don’t see how requiring an Elector to vote for the popular vote candidate does.