The relativity principle was named and popularized by Poincare in 1902, and he believed in it when no one else did. He attributed it to Lorentz, but it is not clear that Lorentz accepted it for all velocities until 1904. Einstein used it in 1905 without mentioning Poincare, and said that he was promoting it from being the “Principle of Relativity” to the status of a postulate. These facts are commonly explained in the history of special relativity.
But what no one explains is that Poincare and Einstein meant very different things by the principle. Einstein meant the same as Lorentz's 1895 theorem of the corresponding states, and Poincare meant covariance. Minkowski's 1908 version also meant covariance, and after that, the Lorentz-Einstein version became just a historical footnote. No one has used it since.
More than anything else, it is this simple fact that has caused me to lose respect for Einstein scholars. They publish involved explanations of why Einstein should be credited, but they do not even recognize what Einstein meant by the relativity principle.
It is the covariance that makes time the fourth dimension and makes relativity the study of spacetime geometry. It is the most central and core concept in the whole theory. Einstein missed it. Not only is it absent from his 1905 paper, but when he was asked to write a relativity review paper in 1907, he failed to understand it in Poincare's 1905 paper, and did not even mention it. And when Minkowski's 1908 paper on the subject caused an international sensation, he denounced the concept. After Grossmann published a covariant description of gravity in 1913, Einstein published papers (wrongly) saying that covariant gravity was impossible.
As the Walter Isaacson biography says:
Einstein, who was still not yet enamored of math, at one point described Minkowski's work as "superfluous learnedness" and joked, "Since the mathematicians have grabbed hold of the theory of relativity, I myself no longer understand it."Einstein's 1905 version of the relativity principle simply postulated what Lorentz proved. That is what Lorentz, Einstein, and everyone else said at the time. And yet no Einstein scholars ever mention this today.
I explain the history of covariance in in How Einstein Ruined Physics.
Update: Another new paper by the same Hungarians tries to show that electromagnetism is covariant. This looks like standard textbook stuff, made confusing by weird philosophical viewpoints.