We analyze RCD linear scalar perturbations in the quasi-static regime. The homogeneous cosmological background forces all EFT modification parameters to unity: mu = eta = Sigma = 1. RCD is indistinguishable from LCDM at linear scales, with Delta (f*sigma₈) /f*sigma₈ < 1e-6. Modifications activate only in the non-linear regime where coherence gradients become non-negligible. This linear safety is a structural consequence of the action, not a tuning.
Arturo Cerezo (Sun,) studied this question.