Charles Rosen wrote that "Sonata form could not be defined until it was dead". I always thought it fascinating that there was no definition of the form until long after Haydn/Mozart/Beethoven were dead. Beethoven did not have a theory book in which he could look up what to do. He was just carrying on a tradition and extending it to suit his own purposes. I doubt that he felt the need to conform to what other people had done - and there were no written rules for him to break. But he was a composer of his time, so he did something that was familiar to him, just with his own individual twist on everything.
So, instead of trying to make the first movement fit a description of a form invented long after the fact, and that is more often wrong than right, I would rather say that no composer ever did more with less than Beethoven. He took some ideas that he presented in the slow introduction (or prelude) and used those as the raw material for the rest of the movement. I am not surprised that he did things differently from other composers, or didn't conform to the post-dated definition of sonata form. It is close enough to what we think of as Sonata form that we can make it fit.
