To play devils advocate I think the conservative justices are trying to draw the line between presidents who are doing illegal acts out of necessity or due to circumstances that requires a president to act versus presidents that are committing illegal acts for personal reasons.
For example; let’s say the potus ordered the killing of 200 Americans on American soil. How many of you would say he should be immune from any charges? Probably none of us. Now let’s say the potus made such an order because those people were on a plane that was hijacked and appeared to be headed to a major population center where the potential of thousands of people could be killed. How many would say they should be immune from prosecution for such an order? I’m guessing most of would be.
So there appears to be a line or requirement to invoke such immunity.
That being said, under the circumstances Trump faced there was absolutely zero obligation that he needed to create a fake elector scheme or to overthrow a democratic election, so even under the weakest of presidential duty tests, trumps actions would fail to meet those tests.