Jack Smith is not claiming that Presidential Immunity doesn’t exist. His claim is that President Trump isn’t entitled to immunity. That Trump should be denied immunity based on doing something Smith didn’t like.
Immunity by definition is black and white. Does an office holder have it or not. In the case of the President, answer is yes. The Office of the President does have immunity.
Smith seems to believe that our founding fathers wanted an unelected federal prosecutor to decide who should or shouldn’t have immunity, and could sit in the office.