[CraigS]

So, after reading the paper, the points they are making are:

Gravitational wave detectors, such as LISA, will enable the use of gravitational wave signals from Massive Black Hole, (MBH), mergers as a new, independent probe of cosmic structure formation (galaxies, etc).

Gravitational waveforms can allow the determination of mass, spin, and orbital parameters of the merging MBHs.

This information could be used to infer the masses of the merging host galaxies. However, their results demonstrate that this connection cannot be made by simply applying the observed scaling relations between the masses of MBHs and the properties of their host galaxies, even if these scalings are applicable to galaxies throughout cosmic history.

Their findings suggest that the mapping between galaxy and MBH mergers depends on various factors, such as the gas content of the merging galaxies and the encounter geometry, and as such might need to be approached in a probabilistic way.

... Hmm ... 'truing' up the models & scaling-up factors underpinning present galaxy origins ..

Cheers