First - if I order an enemy of the US dead I can be prosecuted.
The president orders an enemy dead. That enemy is killed. The president cannot be prosecuted for that act.
What this ruling does - the president may also not be prosecuted for that act after they leave office.
That’s all this does. That’s it. If the president kills a maid in the White House he or she will go to prison because that is against the law and not within the duties of the office.