[aidenn0]

This arises from a parameter in the elementary field equation. If that parameter is non-zero than it is both true that the field is stiff and it must be mediated by a particle with non-zero rest mass. This says nothing about causality.

[scotty79]

Correct. The author explains how both limited range and particles ability to have energy even at rest comes from "stiffness" (which is how he decided to call this parameter you talk about, in order to convey physics to laymen).