This was already the arrangement. That's why Trump was even at the Supreme Court. He was asking for them to decide that everything he did as president was an "official act". They gave the right to decide that back to the lower courts, where it could theoretically come back to them with a more specific set of actions that they need to decide upon.
Of course, the idea that the SCOTUS is corrupted to the point that they would protect Republicans and sabotage Democrats is a worth discussing, but that seems like a wholly different issue that we allowed the highest court in the country to be corrupted by overt partisanship.
It doesn't seem so much that the claim is that the SCOTUS gave Trump immunity, but that nobody trusts the court system to draw that line to begin with.