That's not entirely true... the ligs are only worked when stretching from 9:00 (to a lesser extent) and below. Stretching above 9:00 does little to nothing for the ligs so the stretch is directed to the shaft (tunica).
Think about how the lig attaches the
penis to the pubis ... it attaches the penis from the topside so only downward stretching will put direct tension on the ligs. Stretching upward pushes the penile shaft into the ligs and pubic bone negating most of the tension applied.