I did read it. The claim is to lower the impedance of the crossover as seen by the driver to prevent higher distortion.See section 1.3. You want to alter the actual impedance not just the SPL of problem area.
In configuration #2 the 5 kHz series notch filter increases the impedance. In configuration #3 the 10 kHz parallel filter does decrease impedance but it's still at least 300 mOhm (L3) and in series with the 5 kHz notch filter. In an active speaker the driver sees only the output impedance of the power amp which is easily 10 times lower than L3.