I think that's a bit unclear. They have this standard where things that are part of a presidents "core constitutional powers" enjoy absolute immunity and anything else enjoys "at least presumptive immunity". I think this hints that some members of the court wanted to go further, but anyway I'm not quite sure what "presumptive immunity" means in the context of the law, but I think this quote gives the most clarity for me.
> unless the Government can show that applying a criminal prohibition to that act would pose no “dangers of intrusion on the authority and functions of the Executive Branch.” Fitzgerald, 457 U. S., at 754. Pp. 12–15.
This seems to be the real boundary. To show something doesn't have immunity you either have to:
1. Show it was an unofficial act
2. Show it wasn't part of the core presidential powers and that prosecuting it wouldn't have any danger of intruding intruding on "the authority and functions of the Executive Branch."
I think that last standard is related to presidential immunity from subpoena which courts have also recently construed quite broadly
The part of the text you quoted that really sent shivers down my spine was:
> Nor may courts deem an action unofficial merely because it allegedly violates a generally applicable law.
So now the president can clearly break the law, and he cannot be questioned about it if it even remotely looks like maybe it could have been an official act? Surely, the correct ruling wold be that anything illegal is intrinsically not an official act, otherwise we wind up where the institution of the government is not bound by its founding documents. The executive branch of the government is a rogue entity with nothing to restrain it, as long as the president remembers to use a pompous looking stamp. For *certainly you can never show it was an unofficial act if you are not allowed to investigate because it is presumed an official act subject to absolute immunity *.
Mark my words. The next republican president will shut down any and all attempts to even question him or view documents, by preempting the investigation using this as his argument. And shortly later they will, most likely, start disbarring and / or incarcerating anybody who attempts to investigate him for "frivolous" persecutions that impede the executive branch. This ruling is very calculated, since they know Biden will not abuse his powers because of it, but there is an almost unstoppable amount of latent power that will be immediately abused by the next president.
> unless the Government can show that applying a criminal prohibition to that act would pose no “dangers of intrusion on the authority and functions of the Executive Branch.” Fitzgerald, 457 U. S., at 754. Pp. 12–15.
This seems to be the real boundary. To show something doesn't have immunity you either have to:
1. Show it was an unofficial act
2. Show it wasn't part of the core presidential powers and that prosecuting it wouldn't have any danger of intruding intruding on "the authority and functions of the Executive Branch."
I think that last standard is related to presidential immunity from subpoena which courts have also recently construed quite broadly