I'm not understanding how immunity means you can't use what he does as evidence. Wouldn't immunity mean charges can't be brought against him for what he did in office, not that what he did in office can't be entered as evidence?
In a sane world, sure. Unfortunately that was not the SCOTUS ruling here, which is that for 'official acts':
1) the president from all criminal charges
2) his motive for engaging in those 'official acts' cannot be questioned, no matter how obviously corrupt
3) any 'official acts' the president takes cannot be used for evidence against him for any other crime
So basically if the president DID go order Seal Team 6 to assassinate his political rivals he can't be prosecuted for it, you can't ask why he did it, and if he was talking it over with the Secretary of Defense beforehand about how he can't wait to murder his rivals that cannot be used against him in any other proceeding.
You're probably thinking 'there's no way that's right, that's way too nuts' but, well, it is real.