An assassination isn’t “an official act”, plain and simple. The Supreme Court ruled on one specific case. They allowed it then. A different case could be ruled illegal. Which it would 100% be done, be it a republican court, a democratic court, or some magical unbiased one from fairy land.
The decision they made wasn’t a precedent, they allowed it as a case-by-case basis. So Biden wouldn’t be able to point to Trump and say “he did it! So it’s legal!”.