Complex sentence: This assumption is not true if one-way light speed is anisotropic and employ the Einstein synchronisation, because the transitivity condition cannot be met for observers who are not standing on a straight line.
Split into simpler sentences: This assumption is not true if the one-way light speed is anisotropic and we employ the Einstein synchronisation. This is because in doing so the transitivity condition cannot be met for observers who are not standing on a straight line.