They would not be able to sue Biden PERSONALLY over his official acts but they could still sue the government. That's why the go-to plan here is to have anyone who attempts to sue or any judge that rules against Biden killed.One of Donny's main angles in pursuing the ruling was in the case with Carroll. He claimed his words denigrating her while he was in Office were an official act and thus he should not be subject to such a lawsuit.
But in fact, those words were not an official act in an sense of the term.
Joe's order to divest the companies in the interest of National Security would be an official act, so at least in the Donny sense, they could not, should not be able to sue.
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Biden commanding Seal Team Six to assassinate them is an official act as he's giving orders to the military and while his assassination squads wouldn't have immunity themselves Biden can just pardon them. The main drawback for Biden is he would need to recruit assassins who wanted to live in DC as if they lived elsewhere a state could prosecute the murder.
The 'official acts' ruling doesn't mean Biden can enact any policy he wants and have the courts follow it - what it does is make it so Biden can kill any judge that rules against him without consequence.