> has the authority to issue lawful orders

This decision says that he can issue unlawful orders, and there's nothing the court can do about it. He's immune. You don't need immunity for lawful acts. The very best you could argue is that this prevents prosecution for "gray area" acts that may or may not be lawful. But this decision essentially says that all of those "gray areas" are effectively lawful.

Decide that the protesters in Minnesota are an insurrection? Maybe they start turning up with long guns, like countless previous protestors? Order the troops to fire. It's up to them if they do or don't, but it's guaranteed if they don't, they'll be in courts martial for disobeying the order. The meeting minutes, the reports, what was known and when it was known, Trump's motive: all of them don't matter at all. The official records are inadmissible, his motive is unquestionable, and he is absolutely immune for his orders as commander-in-chief. He can pardon everyone and make them federally immune as well. Only state courts can do anything, far after the fact.