Huh? What a curious misunderstanding! The theoretical referred just the - theoretical! - question of whether it's in principle possible to acquire native-like proficiency, which was contrasted with my claim that even if it is, most people cannot expect to reach that state in practice.
I thought that my choice of the word "commonly" indicated that I was not talking about the limits of the possible.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.