Er, I don't really understand your objection. In \Lambda-CDM General Relativity is also postulated, as are general covariance and gauge theory. Each of these in turn rest on other postulates. All of which are testable within some limit, many of which have already been confirmed to exquisite precision. If compelling evidence arises that is incompatible with any of these postulates, \Lambda-CDM would cease to be a concordance cosmology.

> someone ... created [a theory] that assumed ... dark matter

Yes, that's what theorists do. Then their theories must confront evidence from direct experiment and astrophysical observation. Most theories fail very hard and very quickly; there are an awful lot of different lines of evidence from modern experimental physics and astronomy, and all of them have to be met in a concordance cosmology.

Indeed, the article at the top highlights that a "hm, that's odd, I can't explain it if the observations hold up" will disfavour a family of serious alternatives to \Lambda-CDM. \Lambda-CDM is just fine with galaxies having substantial underdensities and overdensities of dark matter, since the coupling between CDM and luminous matter is very weak. More galaxies like the one in the article, and indeed their opposites where there is a lensing event with surprisingly little luminous matter, would be more evidence supporting CDM.

On the other hand, a cosmology which does not postulate General Relativity and instead proposes a modified theory of gravitation wherein gravitational interactions are sourced exclusively by luminous matter (and the six non-luminous matter particles already in the Standard Model) cannot be a concordance cosmology in the face of a zoo of galaxies like the one in the article.

> a theory that predicts dark matter from some other set of assumptions

Cold Dark Matter was driven by a set of large scale observations starting in the 1960s. Other lines of evidence, starting in the 1990s (BOOMERaNG experiment) also began to demand it at wholly different scales. What was not demanded was any particular microscropic description -- CDM was an "in the large" matter field with some particular characteristics in the Friedmann-Lemaître-Robertson-Walker model, and could in principle be a large mix of types of non-luminous matter including a large fraction of merely hard-to-see isolated Jupiter-esque objects made of ordinary Standard Model particles.

Entirely separately, the application of gauge theory to solve problems in the Standard Model -- wholly unconnected to the gravitational sector -- suggested the addition of extra particles, several of which could in large number fulfil the large-scale requirements of Cold Dark Matter. "Oh, neat, our proposal for a sterile neutrino or a lowest-mass superpartner, or an axion can behave like Cold Dark Matter, let's look for astrophysical evidence of our proposed particle, as well as evidence from laboratory experiments".

One can consider the reverse: neutrinos were proposed before much was known about galaxies (and even directly detected years before galaxy rotation curves were studied by Rubin et al.). Lots and lots and lots of them are produced in common astrophysical processes like stellar nucleosynthesis, and thus would have to enter into the total energy density of a cosmological model (and they do, as Hot Dark Matter, as a component of the \Lambda-CDM parameter \Omega_rad). Lots and lots and lots of them are also expected in cosmological nucleosynthesis (baryogenesis), and so also have to enter into the total energy density (and they do, as relic neutrinos of the Cosmic Neutrino Background, parameterized as \Omega_\nu). As we discover more about the microscopic details of the total energy-density, more components of the total \Omega are likely to be added. We are in serious trouble if we have to remove an existing component, however, or if we cannot resolve conflicting evidence for the total energy density \Omega_tot.

A modified gravity theory that produces a cosmology without \Omega_c h^2 ~ 0.12 (the parameter and value representing dark matter density) will tend to struggle with the lines of evidence other than galaxy rotation curves that support the current value.