“But under our system of separated powers, the President may not be prosecuted for exercising his core constitutional powers, and he is entitled to at least presumptive immunity from prosecution for his official acts.”
He's not being prosecuted for exercising core constitutional powers or official acts. He's being prosecuted for election fraud, inciting an insurrectionist mob and mishandling classified documents. None of those are core constitutional powers and they clearly can't be official acts.
Edit: I just love this part-
Without immunity, Trump's lawyer said, sitting presidents would face "blackmail and extortion" by political rivals due to the threat of future prosecution.
Trump just faces blackmail and extortion from his political allies. Like Vladimir Putin.