Had Trump been convicted in either of his impeachments, his acts while President would be prosecutable, but he wasn’t. Biden, however, can still be impeached and convicted. Thus, SCOTUS has given the sitting President no more power than was already afforded the office. Their decision simply pays off Mitch McConnell’s “kick the can” gambit in shielding Trump from legal action. (Not that it was much of a risk given MM had packed the court.)