The "depth" refers to the strike's ability to bypass the protective wall of the rectus abdominis. This is achieved through trajectory (upward or twisting) and fist formation (often using the second knuckle for piercing pressure).
A belly punch, however, targets the soft triangle: the solar plexus and the liver. deep belly punch
and other martial arts communities suggest these alternatives: Medicine Ball Rotational Throws: Translates core strength into actual punching power. Planks & Side Planks: The "depth" refers to the strike's ability to