Did you read the fucking dissent? That's a sitting SC Justice saying that quote, not some arm chair IANAL basement dweller:
“When he uses his official powers in any way, under the majority’s reasoning, he now will be insulated from criminal prosecution. Orders the Navy’s Seal Team 6 to assassinate a political rival? Immune. Organizes a military coup to hold onto power? Immune. Takes a bribe in exchange for a pardon? Immune. Immune, immune, immune,” Sotomayor wrote.
If one of the dissenting justices thinks it likely, we better pay attention. The whole "They were very clear that non-official acts are not covered." is a pillar built on sinking sand - what defines non-official becomes subjective real fast. Biden could assassinate every conservative justice on SCOTUS and get his own in there to make it all legal. Threats of the same to any in congress who won't play ball.
And if someone can't imagine Biden doing it (I can't), I'm thinking that there are quite a few citizens who believe Trump abso-fucking-lutely would pull that shit. With a majority on SCOTUS already he could just start going after political rivals and keep SCOTUS themselves in check with threats of the same. If SCOTUS has done anything they've painted themselves in a corner and only Congress can unfuck us with impeachment (as unlikely as that seems!)