I'm not sure what point you think you're making but no one is talking about indicting a president for presidential acts.

Oh, but we are.

Yes, we all disagree with Trump that these acts were official or presidential. But that's not how the lower courts decided the issue. Trump alleged at the trial court that all of his acts were official, and therefore immune from prosecution. The lower court could have disposed of that defense by having an evidentiary hearing and ruling that the acts were private, and therefore not immune. Instead, the lower court ruled that criminal immunity was not available for any acts. The appellate court also held that immunity was unavailable for any acts, even official ones.

So that's how the issue got presented to the SCOTUS - whether there is any immunity for any actions by the President, official or private. So the Court has to rule on both buckets of actions. Because if immunity does exist for presidential acts, then Trump has the right to make the argument that his acts were presidential. The lower courts never ruled on whether Trump's acts were official or private, the Court doesn't have that in front of them - all they can do is establish rules that the lower court has to follow when making that determination, if or when they draw a distinction between presidential and non-presidential acts.