A first continuation with Van Flandern's objection: Lorentz/Poincaré/Einstein synchronization (for the history see https://en.wikipedia.org/wiki/Lorentz_ether_theory#Local_time) must be considered a de-synchronization rather than synchronization. Indeed, as long as we restrict to two "material points" A and B without a relative to each other velocity v, corresponding to Einstein's coordinate system at rest, there is no reason for synchronizing A and B by means of light. In a thought experiment, we may move clock A as slowly as desired, i.e. with v-->0, to B.
Einstein gave the following condition of synchronism t_B - t_A = t'_A - t_B which means that the time of flight is the same for AB as for BA. In what Einstein called the stationary case, his equation in [10] is still "free of contradictions" even "for arbitrarily many points" (in Einstein's stationary case):
2AB / (t'_A - t_A) = t
However it is obviously not generally valid, not with v =|= 0. In order to decide to what extent Lorentz, Poincare, and Einstein just speculated, we may notice Einstein's wording:
"we assume that", "we postulate" and "with the help of some physical (thought) experiments we have laid down ... and obviously obtained thereby a definition of synchronism and time".
Einstein gave no references and didn't perform experiments.
EB