That's not true actually. Adding polyfil doesn't affect the effective line length. It will help dampen the response and smooth out upper harmonics. Transmission lines will have an oscillating higher frequency response with peaks and valleys. Adding damping to the line will help average those peaks and valleys and reduce the upper order harmonics and thus lowering distortion and increasing sound quality. Stuffing too much will have negative results though that could leave you with a loss in output and low frequency extension. Damping will also reduce output around resonance as well. It will also help flatten your impedance curve and draw the two peaks together to make one low Q peak.
Adding polyfil to a line that is too short will not help correct the problem.
