That's the "linear-no-threshold" model (LNT) and I see no attempt to show that it holds here. They jumped straight to exposures thousands of times higher than typical heavy use, which smells like they were fishing for a positive result.
LNT is a theory of ionizing radiation, and while your overall point is salient, the reference to LNT is not. While many things enjoy a similar exposure model, LNT is a specific term.
