That is presenting the facts upside-down! At the risk of SRT being deleted by "mainstream", here's the "principle of the constancy of the velocity of light" as reiterated by Einstein in 1907:What do you means is made?
Light is isotropic in ECI otherwise, you could not synch clocks in it. This is a well know fact in the mainstream.
"We [...] assume that the clocks can be adjusted in such a way that
the propagation velocity of every light ray in vacuum - measured by
means of these clocks - becomes everywhere equal to a universal
constant c, provided that the coordinate system is not accelerated.