Who cares? It wasn't contested on Second Amendment grounds. It was contested on the grounds that BATFE violated statutory law.
They put the ball back in Congress’s court where it belongs.
No, it doesn't belong in Congress's court. Congress is explicitly forbidden to infringe the right to keep and bear arms.
Congress outlawed full auto rifles. Whether or not they can outlaw bumpstocks is a matter that the court will have to decide, if and when it comes up. A fully automatic AR-15 is less dangerous than an AR-15 with a bumpstock.