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.
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.