I'm a little confused. Isn't their ruling just a deferral back to lower courts?
They didn't grant him absolute immunity, they just reaffirmed the incredibly broad language in Article II Section 3 of the Constitution.
They're not giving him immunity for everything he did as president, they just aren't interested in being the authority that decides what is or isn't an "official act". They are letting lower courts decide that.
If there's something I'm missing here, I would love to know, but it feels like people are misunderstanding this decision en masse.