Not sure honestly. That might have something to do with the vHDM theory mentioned in the paper. I didn't really follow a lot of it, but I think (major grain of salt here) it's saying that that theory predicts some very light neutrinos exist and tend to collect inside of galactic clusters, but not galaxies themselves and that it's those neutrinos that make up the missing mass at universal scales that general relativity explains using dark matter.
Either case seems pretty hand wavy honestly. When it comes to galaxy and universe level physics it all seems pretty weak compared to the sort of particle physics and classical physics that we can actually measure and test on Earth. It's all just a bunch of theoretical math with relatively few actual measurements to pin it all down. I don't think we're anywhere near having a solid theory of the universe so it's mostly an exercise in trying to prove which theory is the least wrong at this point, rather than which one is correct.
The bullet cluster is an n of one and the dark matter estimates come from redshifting/lensing, which could have come from other sources in such an unusual scenario.
Some of the other crazy observations like ultradiffuse galaxies are, on recalculation, not as extreme as initially guessed, and predicated on an indirect empirical estimate of mass (number of globular clusters) with no mechanistic confirmation.
Either case seems pretty hand wavy honestly. When it comes to galaxy and universe level physics it all seems pretty weak compared to the sort of particle physics and classical physics that we can actually measure and test on Earth. It's all just a bunch of theoretical math with relatively few actual measurements to pin it all down. I don't think we're anywhere near having a solid theory of the universe so it's mostly an exercise in trying to prove which theory is the least wrong at this point, rather than which one is correct.