“They could have just declined the case” is the overly simplistic but misleading version of Supreme Court law and practice being fed to laypersons by many celebrity television commentators who were rushed on air to breathlessly critique the Court’s grant of certiorari to Donald Trump’s presidential immunity petition.
But they’re not telling you the whole story.
Yes, the Court COULD have declined to take the case. But that’s only part of the story. Among the things the talking heads aren’t telling you is what would happen if the Court DIDN’T take the case.
They could also meet and confirm the lower court’s ruling within a week if they wanted to. They’ve made such fast decisions before when circumstances required it.