Planars have a flat impedance that doesn't vary with frequency. Unlike dynamic headphones which have an impedance that changes with respect to frequency.
The output impedance of an amp forms a voltage divider with the impedance of the headphones. If headphone impedance changes with frequency, you get a different voltage divider at different frequencies, changing the headphones loudness at different frequencies. The 1/8 rule makes these changes small enough that they can't be heard, but there is no change in the voltage divider with Planars so it makes no difference.