Nobody says the CI was responsible for the achievement of the young man, but I can speak from experience that a CI makes being in a learning environment much easier and more stress-free. The constant game of "playing catch up" wears you down, because you can either try to pay attention to your interpreter, but you miss what's being written down on the board (yes note-takers help, but unless you are able to write it down WHILE you are learning it, in some classes like mathematics, or other sciences, reading the notes after the fact are just as confusing as reading them without having listened to a lecture at all,) or you write down everything that the teacher is writing on the board, but you miss out on the explanations on how it works together....I was constantly stressing myself out because there was always *something* that was missing.
I know I know I know (because I know you're going to bring it up,) that there are many deaf students who don't have an issue with that, but I know just as many who do. Please don't try to turn this into yet another "deaf people can do it without the CI, so they don't need it anyway" argument. Heaven forbid anybody do anything to make a child's life easier.