The speed of light is constant, even in matter. The ‘speed of light’ in a transparent material is just the net perceived macroscopic interference wavefront from a vast number of individual absorptions, excitations, and emissions of the constituent atoms in the material. Light travels the vacuum from atom to atom at c. Also, “the speed of light” is a much deeper concept, and is a structural constant of spacetime, and exists independent of the particular properties of photons. In a universe without electromagnetic radiation, c would still exist, but would not be quite so convenient to measure.
Isn’t related to the properties of the E and the M in a TEM wave and how the two reinforce/produce each other at a fixed rate which is c?