Standard gravitons are already constrained to be almost massless, so i'm guessing the article is referring to Randall-Sundrum type massive gravitons in 5D.
Why does their detectability depend on the mass? My understanding is that both the standard and Randall-Sundrum gravitons can only be identified through missing energy; neither are possible to directly detect through any feasible experiment. Does the low upper bound on the mass constrain the cross-section?
Is the following reasoning correct? If the standard graviton is massless, it can't decay into r rbar for any particle r (because of momentum conservation). The standard graviton could have a tiny mass, but the low upper bounds on it means that the cross section is too tiny to be observed. RS gets around this because at LHC energies the RS graviton has a larger high-energy effective mass (because of extra dimesions) while still satisfying the low-energy experimental mass limits.