That may be the case. However, they still assume c is a constant, in the same way as they also assume that gravity is the prevalent force even in areas that have high plasma reactions. The normal assumption (in all the papers I have seen to date by the "conventional" cosmologists) is that the universe is electrically neutral and hence the only significant force is gravity.
This has led to the idea that there has to be additional unseen, unmeasured matter (aka dark matter) and unseen and unmeasured energy (aka dark energy) to explain the anomalous measurements of galactic movement that is not explained by the "gravity is the only significant force" model.
Now, it may be that there is such a beasty. But, I have my serious doubts because electromagnetic fields have more intense effects than gravity, by some orders of magnitude (about 36 orders of magnitude).
In other words, some environments may be completely neutral and gravity is the major effect, in others, the environment may be very electrical and gravity is the least effect.
Our problem is that we can observe only at vast distances and are not able to take direct measurements to confirm our indirect modelling measurements.
Science is no different to any other aspect of people's lives, they will invest great amounts of energy in following their specific model of the world. When challenged, they mostly look at other as imbeciles who are incapable of seeing "the truth". I've seen this occurring in areas like string theory, black holes, dark matter, sub-atomic physics, climate science, immunology and vaccinations, medical surgical techniques, religion, politics, economics, sociology, finance and business, education, archaeology, evolution, etc., etc., etc.
Our models and theories are just that models and theories. We hope and believe they give us insight into the universe around us, but they are not reality. Each has limits and all use simplifying assumptions that are not accurate outside of the limits imposed by those assumptions.
If on the basis of our assumptions, we have to keep adding more and more "strange" entities to make our model fit the observations, then, maybe, we should go back and look at the underpinnings of our models and see if another model will work better (as in the kind of change that eventually occurred when going from the earth-centric epicyclic model to the sun-centred heliocentric model).
The fun is being able to learn more and more about how our universe operates and what we can do within it.