That's not what the Supreme Court said. They said that virtually anything the president does is possibly an official act, and therefore can't even be prosecuted, unless the prosecutors first prove to a judge that it wasn't an official act. And if the judge isn't convinced it wasn't an official act, then the president is immune from any prosecution for that act. So if Trump ordered the military to kill Harris and Obama, this would be an official act and he would be free from any legal consequence for it - the judiciary couldn't even investigate the reasons for it, because that would encroach on the power of the executive branch.

However, this laughably broad, king-level power still doesn't grant him the power to just make anything into law. He can't be prosecuted for anything, but that doesn't mean that anything he wants is now law.