To summarize, Dr. Song started out with a wing that has low drag at supersonic speed but performs poorly at subsonic speed/high AoA, and augmented the heck out of it with lift. The first obvious feature is LEX, then comes canards, and finally lift from body. In the end, through the interaction between LEX and canards, he was able to get even more lift by reducing* the wing's aspect-ratio, even though the conventional wisdom is that the larger aspect-ratio the better. The presence of LEX enables the canards to be placed far from the wings yet provide similar lift enhancement as close-couple canards. Canard configuration is also chosen because in the absence of TVC, canards provide total pitch authority at maximum AoA whereas elevators cannot.
*Obviously, the J-20 is an unconvential design, and cannot only be analyzed with classical aerodynamic theories.
