There is no wording about congress at all regarding an official act.
Official act" for the purposes of Section 201(b) and (c) is defined to mean:
"Any decision or action on any question, matter, cause, suit, proceeding or controversy, which may at any time be pending, or which may by law be brought before any public official, in such official's official capacity, or in such official's place of trust or profit.” https://www.justice.gov/archives/jm/criminal-resource-manual-2044-particular-elements
in the ruling they just use the words “ core constitutional power” but according to this ruling the president has immunity so he could theoretically sign an executive order giving him extra powers and legally there’s nothing could be done because that had nothing to do with congress it was done by executive
In theory Biden could sign an EO suspending habeas corpus and that’s within his new immunity rights