I think if the Court was concerned the DC Circuit decision was too broad, and felt like it needed to take cert, the right approach would have been an expedited decision that said, whatever the bounds of presidential immunity, this is outside it. Not a decision "for the ages"
There's a good case for what you might call strategic ambiguity: you don't want presidents to know they can't be prosecuted but you also want to deter prosecutions that stretch to cover legitimate (even if ultimately unlawful) discretionary decisions. Trump was way outside the bounds. Just say that
