I am not an expert on dark matter, but:

Attempts at including this effect have been made before, as noted in the paper. The author notes that those attempts underestimated the magnitude of the effect by applying an oversimplified model.

The paper is by a plasma fluid-dynamics expert more familiar than your typical astrophysicist with the necessary mathematics. The mathematics of General Relativity are extremely difficult to work with, so it is almost always necessary to use a seriously simplified version. It takes a lot of experience to know how much simplification you can get away with and still get a valid result. It takes even more to work the less-simplified model that it turns out is needed.

Fortunately, plasma physicists have been obliged to develop that experience to ever get any useful result with plasma.

If this result checks out, it will square General Relativity with rotation rates of at least some galaxies, leaving no need to fudge them with Dark Matter. That would mean that these galaxies cannot have anywhere near the amount of dark matter in them that has previously been assumed, and could in fact have none.

If very many galaxies qualify, the total amount of remaining dark matter becomes too small for what has been attributed to it.

There are a variety of other phenomena fudged with Dark Matter, not least the currently favored cosmology. It needs a hell of a lot more matter than will fit in all the visible stars and interstellar and intergalactic plasma (what they like to call "gas"). If the galaxies don't have this matter, they need to find some other place for it to be, or come up with another cosmology that needs less matter.

Gravitational lensing is a whole other topic. If gravitational lensing with galaxy clusters demands more mass than the cluster's galaxies appear to have, the dark matter has to still be there, just not clumped close around the galaxies, but instead, I guess, more spread out throughout the cluster.

(I would guess that this would imply much less massive dark-matter particles. It was my impression that such low-mass particles, if they obey understood physics, had been ruled out by existing data. Maybe they just cool much more slowly, instead?)

It might be that some phenomena we thought were "Accounted-for" will need to be moved back to the "Mysteries" column. Most likely, regular astrophysicists and cosmologists will fight tooth and nail to avoid confronting any of this, and numbers of them will need to retire first. The response, when it is finally obliged, will almost certainly be another incremental adjustment to existing lore, and insistence that we ignore its predicting both before- and after- dark-matter models equally well.

Now, It Seems to Me, in my ignorant condition, that a numerical GR simulation of an actual galaxy, with initial condition just the assumed regular orbital motion of all the, say, 10^11 stars and accompanying plasma interstellar medium, run for a just a single time step, computing the motion, in that time step, of just a single star, would enable bypassing all the intractable maths and delivering a precise and accurate readout of the motion of that star. I.e., does it continue accelerating in its elliptical path, bent to it by General Relativity spatial warpage, or diverge? I would welcome learning why this would not work, or why it has not been done, because it seems like it could be tried in hardly any time at all.