Nitpicking: It is not possible to derive the theory of relativity from medieval knowledge. At least not in any way resembling how the development actually happened, that is, heavily influenced by observations (or rather an iteration of speculation and observation). [Unless one counts both Galilean relativity and some parts of the theory of electromagnetism as being contained in medieval knowledge.]

Perhaps it would be possible if the classically observed reality turned out to be possible (consistent under some reasonable conditions) only as a consequences of some deeper, sufficiently determined theory; but that would go far beyond our current level of knowledge, at least as far as I can say.

The earliest you could reasonably derive special relativity is probably 1881 with the first version of the Michelson–Morley experiment, which showed that speed of light is the same no matter how you move through space. Which, on the scale of discoveries, is a pretty short timeline to the 1905 publication of special relativity.

Which doesn't really stop you from training an LLM on 1904 knowledge and having it derive special relativity. LLMs trained with knowledge cutoffs that far back is a fun exercise, and one I've also dabbled in in the past. But you don't have anywhere near enough training data to reach SotA levels of intelligence, even if you had the money for those training runs. And lobotomizing all modern information out of LLMs doesn't seem viable either. So I don't see how you could ever turn that into a viable benchmark