@3ammo your use of words are loaded here.. you are correct that a lorentz transformation assumes the instantaneous speed of light is a constant in all frames.. but it does not derive from that as a **postulate** it derives from that as a **convention**. It is an important difference. If it were a postulate it would assume it makes some meaningful difference, and states something about how the world **is**... the symmetry of C is not of this nature. Because the symmetry of C is a convention, used simply because it gives you the same experimental results but is simpler, and not an assertion of reality, then the lorentz transformation is derived while using the **convention** that c is symmetric.

To put it another way, you could, if you wanted, pick any arbitrary asymmetry for the speed of light that preserves the round-trip speed of light as C, and then re-derrive the lorentz transformation based on that (and get a much more complex result).. you can then apply that modified lorentz transformation to any real world experiment and wind up with the exact same results as the much simpler convention... again distinguishing it as a convention rather than a postulate.