Even if everything he said were correct, and Poincare were a senile old fool who constantly babbled nonsense, believed in astrology, and prayed to Satan, it would not matter to my argument. Poincare published the essential ideas and proofs that turned Lorentz's theory into the modern theory special relativity. Einstein did not even understand those ideas.
Einstein was wrong about many things. Hans Ohanian wrote a whole book on Einstein's Mistakes. Nobody denies Einstein credit because he made mistakes. People get credited for what they do right.
LionAxe says that Poincare believed that electromagnetism requires a non-vacuum medium for wave propagation and that the universe has a preferred frame that defines motion. If so, Poincare was right, because today's physics textbooks say that both of these are correct. Electromagnetic waves are explained by quantum electrodynamics (QED), and that requires a nontrivial vacuum state different from the vacuum. And observations of the cosmic microwave background have shown a preferred frame for motion.
The QED vacuum state (ie, modern aether) is Lorentz invariant. If LionAxe could find a quote showing that Poincare believed otherwise, then I would agree that he said something that was eventually determined to be wrong. However, LionAxe's quotes show that Poincare believed that it was impossible to detect absolute motion, and therefore his ideas were consistent with the modern idea of Lorentz invariance of QED.
As quoted, Poincare explained "the reason why we believe in an aether is simple." Poincare argued that the aether was a useful convention, but not essential. He correctly explains the reasons for the belief. He is correct.
Poincare also explained Lorentz's theory, using Lorentz's terminology. Lorentz explained in 1895 why the evidence was against the aether drag hypothesis, and used the term aether at rest as a shorthand for rejecting that hypothesis. Poincare correctly describes that argument. Einstein avoids the issue because he just postulates Lorentz's conclusions while ignoring the analysis that went into those conclusions (and not citing Lorentz).
The Einstein scholars like to say that Poincare believe in true time, and hence he must have also believed in the aether because only clocks in the aether frame would show the true time. Their proof is Poincare's 1904 St. Louis Worlds Fair lecture where he gives a completely correct description of relativistic time and clock synchronization and then says:
The watches adjusted in that manner do not mark, therefore, the true time; they mark what one may call the local time, so that one of them goes slow on the other.If Poincare were claiming that were a true time, then he would be consistent with modern cosmology. The leading model, the Lambda-CDM model, does in fact have a true time defining the age of the universe.
But Poincare is not asserting a true time; he is denying one. It is as if I said, "I went camping and I did not see bigfoot", and then someone accused me of believing in bigfoot. It is hard to argue with such illogical nonsense. But defense of Einstein's priority is usually based on reversing the meaning of simple sentences like this. The Einstein scholars adamantly and relentlessly defend the idea that he wrote the most original science paper ever written, but they can never explain what was original about it, or how all the truly original ideas were published by Lorentz and Poincare.