What does this equation mean?

Before NICER, there was no way to get M and R for a bursting neutron star independent of a spectral line, so the line had to carry the whole inference: assume an identification, extract a redshift, and only then estimate the equation of state consistent with that redshift and some other mass constraint. That order of operations has a structural weakness the paper’s authors state plainly — a line asked to fix the star’s structure has nothing left over to test the physics of the redshift itself, because any discrepancy could always be waved away as an equation-of-state uncertainty rather than a genuine anomaly. NICER breaks that circularity by supplying $z_s$ from a channel that never touches a transition energy. Once the redshift is known first and independently, a credibly identified surface line stops being a way to weigh the star and becomes a way to test whether matter actually falls the way general relativity says it should.