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?
It’s the reciprocal of the square root of the product of the permittivity and permeability of free space. Interestingly, neither of these properties require the existance of the photon and can be measured in the laboratory with modest equipment. Now as the article suggests, that these ‘constants’ aren’t really constant over vast periods of time, then due to things like the wave equation, light would ‘go along for the ride’ and adjust its speed accordingly, presumably over vast expanses of cosmological time. But for our little snapshot, c is for all intents and purposes, rock solid constant as measured from in all inertial reference frames. As an aside, since we don’t know the nature of conciousness and subjective time, and being essentially chemical meat computers, it makes one wonder if we’d even be aware of such a change in c.