Again, at the actual sentencing, his ruling stated an unconditional discharge was "the only lawful sentence that permits entry of judgment of conviction without encroachment on the highest office of the land".
"I can sentence you, but only to nothing" is functionally not being able to sentence him.
Anyway, in agreement with your larger point, the legal analyst at https://youtu.be/4tbaDI7ycrA?t=592 says he believe this SCOTUS would not have allowed a real sentence, so my nitpicking about the interaction of the 2024 decision with the lower court's sentencing doesn't matter much; SCOTUS would have let Trump go either way, and probably a Biden-packed court wouldn't have.
It's just another sign that modern Republicans aren't truly "Constitution-lovers" or textualists, that their leader is only safe because judicial activism invented immunity for him.