That opinion specifically defers to lower courts to define the precise limits of the President's immunity. I refuse to believe that the collective effort of the judicial branch would construct those limits in a manner that enables dictatorship.
It was a clever but blatant way to give their side immunity without giving it to Biden
To get there, we would need to assume that the Supreme Court, including only 3 Trump-appointed Justices, is both unwaveringly partisan and unwaveringly supportive of dictatorship.