Correct, cl+cr = 2cu is the best you can measure. But it doesn’t follow that cl = cr! Assume a linear transformation like αcl+βcr = 2cu, where α and β tell you

Correct, cl+cr = 2cu is the best you can measure. But it doesn’t follow that cl = cr! Assume a linear transformation like αcl+βcr = 2cu, where α and β tell you how the speed of light varies in between left and right. We simply assume that α = β = 1.0, but what if α=½ and β=³⁄₂? Or α=2 and β=0? A long as α + β = 2.0, then everything will look exactly the same.