I think this is unironically how they need to spin it. Convince the Republican base that this ruling is actually better for Biden than it is for Trump by repeating their own false narratives back to them - that the Biden Crime Family will get away with everything. Albeit, the things he could actually get away with are limited to what the court determines is an official act, and given the current makeup of the SCOTUS it’s unlikely that they would side with him even if there were precedent, but he would be still almost untouchable under this new ruling no matter how you spin it.
Have a case against Joe Biden? Sorry, all of that evidence is now inadmissible in a court of law because it happened while he was president. Too bad, Republicans! Maybe if you were to… I dunno, pass a constitutional amendment that revoked that privilege. But oooooh nooooo, that would be horrible! Please, anything but that! All our nefarious plots would be undone and Biden would go to prison!