This biggest caveat is mentioned in the ApJL paper [0] introduction:
> Because SMBH growth via accretion is expected to be insignificant in red-sequence ellipticals, and because galaxy–galaxy mergers should not on average increase SMBH mass relative to stellar mass, this preferential increase in SMBH mass is challenging to explain via standard galaxy assembly pathways (Farrah et al. 2023, Section 5).
I think there are several observational effects that may obfuscate the interpretation -- but I also haven't read these papers in great detail.
First, there is a known age-metallicity-dust degeneracy that can make dusty star forming galaxies look more like red elliptical galaxies. This can bias estimates of the star formation and mass accretion history -- e.g. perhaps supermassive black holes have had more recent growth. Second, galaxies in more overdense regions may harbor faster growing supermassive black holes, and also be more predisposed to later merging and forming an elliptical galaxy. This seems likely true around cosmic noon (z~1-2), when star formation and supermassive black hole accretion activity were at their highest throughout cosmic history.
I also think that the estimated growth was too big to be caused by such systematic errors.
On the other hand we do not have models of a galaxy evolution based on solving equations of General Relativity. Typically the assumption is that Newtonian gravity with minimal relativistic corrections should be enough. But there is no proof that it is so.
Moreover, there were relatively recent papers that showed that better accounting for General Relativity could be enough to explain rotational curves for Galaxies without any notion of Dark Matter and that the need for Dark Matter was simply ab artifact of the assumption that Newtonian gravity can be used at the galaxy scale.
Then there are speculations that electromagnetic forces do play role at the galaxy scale affecting the rate of evolution of galaxies.
So it can be that the observed discrepancy in the growth of Black Holes caused by holes in galaxy evolution models, not by the proposed new effect.
> Because SMBH growth via accretion is expected to be insignificant in red-sequence ellipticals, and because galaxy–galaxy mergers should not on average increase SMBH mass relative to stellar mass, this preferential increase in SMBH mass is challenging to explain via standard galaxy assembly pathways (Farrah et al. 2023, Section 5).
I think there are several observational effects that may obfuscate the interpretation -- but I also haven't read these papers in great detail.
First, there is a known age-metallicity-dust degeneracy that can make dusty star forming galaxies look more like red elliptical galaxies. This can bias estimates of the star formation and mass accretion history -- e.g. perhaps supermassive black holes have had more recent growth. Second, galaxies in more overdense regions may harbor faster growing supermassive black holes, and also be more predisposed to later merging and forming an elliptical galaxy. This seems likely true around cosmic noon (z~1-2), when star formation and supermassive black hole accretion activity were at their highest throughout cosmic history.
[0] https://iopscience.iop.org/article/10.3847/2041-8213/acb704