Because black holes are different sizes presumably they would contain different quantities of dark energy and so some galaxies would accelerate faster in some directions than others.
Dark energy from supermassive black holes? Physicists spar over radical idea
In that model, black holes are assumed to spread out uniformly throughout the universe after their formation. This is typical behavior for dynamical dark energy models, but it's particularly necessary here to evade established observational constraints on the abundance of massive black holes within galaxies. In Appendix B of their ApJ Letter, where the authors demonstrate their model's compatibility with these limits, they say that they "have assumed a uniformly dispersed population in the computation of Figure 3". It is also clear from the referenced figure that this assumption has indeed been made.
(Whether the implied departure of stellar remnants from galaxies is itself consistent with observations is a separate matter that, to my knowledge, has not been addressed.)
