It's testable! And maybe not just theoretically testable, but realistically testable. So they're not just pontificating, nor even pontificating in a mathematically consistent way.
TL;DR: They're assuming that spacetime is not quantized, but is as GR describes it. This produces a consistent theory that predicts that measured mass will fluctuate very slightly.
TL;DR: They're assuming that spacetime is not quantized, but is as GR describes it. This produces a consistent theory that predicts that measured mass will fluctuate very slightly.