Eventually, Albert Einstein (1905) was the first to completely remove the ad hoc character from the contraction hypothesis, by demonstrating that this contraction did not require motion through a supposed aether, but could be explained using special relativity, which changed our notions of space, time, and simultaneity. Einstein's view was further elaborated by Hermann Minkowski, who demonstrated the geometrical interpretation of all relativistic effects by introducing his concept of four-dimensional spacetime.and here:
In 1907 Einstein criticized the "ad hoc" character of Lorentz's contraction hypothesis in his theory of electrons, because according to him it was an artificial assumption to make the Michelson–Morley experiment conform to Lorentz's stationary aether and the relativity principle.[A 25] Einstein argued that Lorentz's "local time" can simply be called "time", and he stated that the immobile ether as the theoretical foundation of electrodynamics was unsatisfactory.[A 26]
FitzGerald and Lorentz looked at the Michelson-Morley experiment, and other experiments, and deduced that the speed of light appeared the same for all observers. This could be explained by a length contraction.
Einstein looked at the work of Lorentz, and postulated that the speed of light was the same for all observers. Then he deduced the same length contraction.
FitzGerald's and Lorentz's works are said to be ad hoc, because they based it on experiment. Einstein's was not, according to this argument, because he based it on postulates.
Einstein did not have the modern geometrical understanding of relativity. Minkowski demonstrated the geometrical view and elaborated on Poincare's concept of four-dimensional spacetime, but it does not appear that Einstein had any influence on Minkowski.
Special relativity did change our notions of space, time, and simultaneity, but not because of anything Einstein said. Everything Einstein said on these subjects was said earlier and better by Lorentz, Poincare, and Minkowski.
Poincare argued several years earlier that Lorentz's local time can be simply called time. Einstein was just agreeing with Poincare.
I cannot correct Wikipedia, because policy favors the so-called "reliable sources", such as Einstein biographies. The fact is that most of the historians favor Einstein. But if you trace Wikipedia articles to the primary sources, you can see that Einstein's contributions were merely expository.
There is something insidious about this whole concept of a theory being ad hoc. Basing a theory on experiment is a good thing, not a bad thing. If an experiment comes along that challenges your theories, like Michelson-Morley did, then finding some way to modify your theories to accommodate the experiment is just what a good scientist should do.
You might say that you don't just want to fudge your formulas to match the data. It is better to have an underlying theory to explain the fudge. But FitzGerald and Lorentz had exactly that. They had a belief that solid matter was held together by electromagnetic forces, and that changes in the fields would contract the matter. Wikipedia denigrates this, because detailed theories for molecular forces were only worked out later. But nevertheless, their beliefs turned out to be correct, and solid matter is held together by electromagnetic forces. The length contraction can be derived from Lorentz transformations and Maxwell's equations, as Lorentz proved, long before Einstein.
But Einstein had a better explanation, you might say. But that is not true. Einstein did not have the geometrical interpretation that is preferred today, and rejected it for years after others accepted it.
No, the rationale for crediting Einstein is based on anti-scientific ideologies, such as preferring postulates to experiments, and deriving theories grounded in experiemnt as being ad hoc.
For details, see my book, How Einstein Ruined Physics, and postings on this blog, such as this 2017 book update.