Interestingly, the paper proposes that the mass of black holes is "cosmologically coupled" to the expansion rate of the universe. This was based on unexpected mass growth in black holes that our current models do not explain with accretion alone. The observations were made through analysis of older elliptical galaxies and gravitational wave signatures. What is being described by the researchers is the hypothesis that emerged from these unexpected observations.

My question is, if this expansion continues to accelerate to the point where only discrete massive objects exist in an expanding spacetime where all other objects have receded beyond the light horizon, would said objects continue their mass growth being coupled with an arbitrarily large, expanding spacetime? If so, what happens next?

It would seem that in this condition the "universe" would eventually consist of permanently displaced, high-density, high-temperature, low-volume objects gaining mass through some unknown mechanism by manner of spactime's continued expansion. Would the hypothetical conditions that altered the state of the proto-universe from pre-Big Bang to post-Big Bang arise in these objects? Would the continued mass growth over arbitrarily long time scales consequently increase the chances of the hypothesized causative quantum fluctuations yielding renewed expansion in any of these objects? Is there a critical point at which these infintesimally low probability occurences necessarily happen due to the continued addition of mass?

Most of the above is just speculation from someone who doesnt know nearly enough about these matters. Nonetheless, it seems the nature of this relationship - the cosmological coupling of black holes to expansion rate - may provide some very insightful avenues for continued research if the signal proves to represent an actual physical phenomenon.