Skin depth calculation for non-sinusoidal currents
I would have thought that the complete waveform that makes up the PWM synthesis of a triangle wave would have needed to be broken down to fourier constituents. That seems to be what is proposed in the answer by Curd in the previous question.

The paper by C. R. Sullivan seems to present an approximation for a specific purpose. You may need to analyze that paper in more detail and perhaps look at the supporting literature to determine if it is suitable for your purpose. I believe that a Fourier analysis of the actual waveform is the only way to get an exact skin depth. That says that the PWM frequency does influence the skin depth. However a method based on Sullivan may provide sufficient accuracy.