I'm extremely skeptical of a 'simple test' which proclaims to overturn decades of scientific knowledge. Why did it take until 2022 for someone to think of this, and not a hungry grad student in the 80s looking to make a name for themselves?
Otherwise, the article actually seems a surprisingly reasonable complaint about being hard to get funding for trying to develop better alternative models which explain observations better.
But let's be careful to not overgeneralize here. The author is working out of Bonn, Germany. I have no idea how German research gets funded or what their priorities are. More importantly, I have no idea for how that funding model compares with the funding model being used by other countries. All I can reasonably ascertain after reading this article is Germany isn't funding research to develop better alternative models. I have no knowledge for what other countries are funding.
Gravity is damn near perfect on the scale of the solar system. A huge number of zeros of precision, makes great prediction, etc. Even blackholes (which is quite the corner case) seems fine with the current theory of gravity until you get down into the small details and the search for a grand unification of quantum and relativity.
However as the scale of a galaxy things break down. So there's at least 2 possibilities. One that the theory of gravity is wrong at large scales, or there's something else involved, like dark matter.
Seems natural that scientist have a working assumption that the theory of gravity is right, but the problems with it are getting more attention. There's not just one competing theory for the next step for gravity.
(1) the observed differences in gravitational forces come from a gravitational field source, not modified equations themselves
(2) The field sources are a form of matter, thus obeying some "obvious" laws like mass conservation
(3) The new kind of matter is also attracted by gravity itself, which even allows for limited predictions of how its mass distribution behaves
And we can only say it's never been observed directly, because there is indeed enough indirect evidence pointing towards its existence.