"That light requires the same time to traverse the path AM as for the path BM is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity." Einstein 1961
Since you dated it to 1961, I really don't know what point you are trying to make.
Einstein wrote that as part of a fictional dialogue with the reader in a pop-science work. But it is not an example of a naked claim resting only on Einstein's scientific authority.
It dates to the 1920 English version of
Relativity: The Special and General Theory and a full quote makes it clear that the speaker is other than Einstein's, even if Einstein agrees to work with that definition, aka
"Distant events are simultaneous if a particular flash of light emitted from the midpoint of the straight line connecting the events reached the place of the events at the time of the events." (My paraphrase)
The argument may be predicated on the assumption, but the physics is always to be judged against Nature. So your quibble is baseless, because it's part of a rationale, not a fiat claim that stands by itself.
http://www.bartleby.com/173/8.html
Indeed in the previous chapter, Einstein describes why the claim is physically reasonable.
http://www.bartleby.com/173/7.html
The next chapter then shows that definition of simultaneity leads to this new thought: "Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event." (Actual quote)
http://www.bartleby.com/173/9.html
The next chapters develop this theory mathematically.
Then in chapter 13, Einstein applies it to 19th century experiments, and it checks. In chapter 14, we have the summary of the previous chapters: "Experience has led to the conviction that, on the one hand, the principle of relativity holds true, and that on the other hand the velocity of transmission of light in vacuo has to be considered equal to a constant c. By uniting these two postulates we obtained the law of transformation for the rectangular co-ordinates x, y, z and the time t ... the Lorentz transformation.
The law of transmission of light, the acceptance of which is justified by our actual knowledge, played an important part in this process of thought. Once in possession of the Lorentz transformation, however, we can combine this with the principle of relativity, and sum up the theory thus:
... General laws of nature are co-variant with respect to Lorentz transformations. ... If a general law of nature were to be found which did not satisfy this condition, then at least one of the two fundamental assumptions of the theory would have been disproved."
http://www.bartleby.com/173/14.html
So the consistency of the speed of light is an
assumption of the argument, but a mere
hypothesis in the physical theory -- a theory which passes many stringent tests every day in our world 90 years since this appeared in print.
If you just included it to sort out Motor Daddy, good luck with that.