You're missing that the Supreme Court is taking the piss and the District Court they're kicking this back to has already done their homework and defined the official acts versus unofficial acts. They're ret-2-go but the Supreme's did their job of punting this until at least October, since that's when they come back from vacation. So when the District Court punts it back up the chain to the Supreme Court, they have to wait for the Supreme Court to reconvene. It's fucking stupid, but it accomplished getting Trump nothing but a legal time-out.
Oh, ALSO:
Testimony or private records of the President or his advisers probing such conduct may not be admitted as evidence at trial.
They literally fucked us out of a ton of evidence with this part of the ruling.