How can he just do this? Is the Supreme Court irrelevant now?
He can do it for several reasons. First, there's no penalty for willfully violating a supreme court ruling. None. Second, anyone who sues will have to work their way through the entire system, from the lowest of courts, all the way up to the Supreme Court. AND they'll have to have standing (I.e. prove that they were "wronged" by Biden's Order).
Lastly, abiding by the Supreme Court is basically done on the honor system. The Supreme Court has no literal and visceral power. They can only work through others. If they say, "Bad biden!" and Biden tells them to get bent (and other presidents have done this in the past), they can't do much unless others (like the military) enforce it for them.