When he uses his official powers in any way, under the majority’s reasoning, he now will be insulated from criminal prosecution. Orders the Navy’s Seal Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune
In all three of those cases, the question as to whether those actions are required or permitted under the law is put to a judge, and that judge is free to rule that they are not. The trial judge is free to rule that they are “unofficial” acts, and deny that immunity.