I don't think it is, to be honest. All three of those examples are clearly official acts. The President is commander-in-chief of the military, and is also responsible for granting pardons. The key piece of the ruling which lends credibility to her examples is that any motive or details behind the official acts are immune from scrutiny.
The only real difference is whether the acts are official or not. So the President is allowed to order the military to assassinate a political rival, but not to pay a private hitman to do so.
It's a pretty ridiculous and indefensible stance from the majority opinion.
They are not clearly official acts. It's going to be years of decisions and debate to define official acts.
As an example, the fact he's in charge of the military doesn't make everything he asks them to do official because he is obliged to follow the constitution. There are arguments to make against what I just said. It's as clear as mud once you start going through concrete scenarios.
This is as the Imperial Supreme Court wants it, so that it will come back to them every time an important decision needs to be made. This enhances their power and ability to shape events. Murkier is better.
It also makes it easier for partisan rulings on a case-by-case basis. If defendant is on your team: allow it, if not, issue a self-contained ruling (or don't pick up the case at all if lower courts ruled against them).
We have front-row seats to how empires decline and fade away. I guess those who missed the 20th century decline of the British Empire (culminating in Brexit) can study this one.
The constitution says very little. Most legislation says very little. I don't think they are grabbing for power, they are just stating the obvious - this isn't covered, so I guess we'll cover it.
The only real difference is whether the acts are official or not. So the President is allowed to order the military to assassinate a political rival, but not to pay a private hitman to do so.
It's a pretty ridiculous and indefensible stance from the majority opinion.