There's a shit ton of questions to be answered in the litigation. But the one this was decided on is what is the proper court in which to bring these challenges.
The Supreme Court decided that the proper court is the one in which a petition for habeas corpus lies, which is a district court in the state and judicial district in which the petitioner is detained.
As a matter of first principles, deciding that the trial court lacked jurisdiction is just the kind of basis that a "shadow docket" decision is based on. But that is the case only where the law is clear on which court has jurisdiction.
But, which court has jurisdiction and whether the petitioner's claims are governed by habeas corpus, only, or some other remedy, in this case the Administrative Procedure Act, is a novel question in this context, never before decided.
So they fucking decided it on an emergency motion on the shadow docket, making new law. Also notable that they explicitly said "venue is wrong." Venue is a relatively minor thing that does not affect a court's power to rule; a court that entirely lacks jurisdiction is powerless, one that is not a proper venue should be transferred to one that is proper, but that can happen later in time and does not invalidate the courts' rulings.
Complete. Fucking. Bullshit.