I'm not sure that SCOTUS's immunity ruling is all that important here. It mainly served to delay Trump's federal criminal trials until it was too late to get anywhere. In the absence of that ruling, a president could still -- in collusion with their VP -- obtain immunity by:
1. While in office, relying on the DoJ's policy of never indicting a sitting president. (Not to mention the fact that they control the DoJ.)
2. On the last day of their presidency, they resign. The VP is sworn in as president, and pardons the now-former president for whatever they've done. (The former president can of course, before resigning, pardon the VP for anything they might have been involved in as well.)
Easy peasy.
Sure, Trump could "find a way" regardless. But the SCOTUS ruling gives him more of a cover, and more importantly, removed a potential threat.
It’s important because it gives him 100% immunity from all actions that even slightly be adjacent to “executive branch” . Almost unlimited spectrum of activities are allowed including killing Americans for no good reason. Short of him walking out the door of the WH and gunning down a tour bus of tourists with his own machine gun, the SCOTUS gave him cover to say “whoops, I thought that was ok”