The main crux of their argument is assuming only advanced alien civilizations use a lot of energy:

Professor Michael Garrett (ASTRON & University of Leiden) has used radio measurements of the very best candidate galaxies and discovered that the vast majority of these systems present emission that is best explained by natural astrophysical processes. In particular, the galaxies as a sample, follow a well-known global relation that holds for almost all galaxies—the so-called “mid-infrared radio correlation.” The presence of radio emission at the levels expected from the correlation, suggests that the mid-IR emission is not heat from alien factories but more likely emission from dust—for example, dust generated and heated by regions of massive star formation.

As Professor Garrett explains: “The original research at Penn State has already told us that such systems are very rare, but the new analysis suggests that this is probably an understatement, and that advanced Kardashev Type III civilizations basically don’t exist in the local universe. In my view, it means we can all sleep safely in our beds tonight—an alien invasion doesn’t seem at all likely!”

Does this sound reasonable?  We are making a lot of assumptions with this hypotheses.  By definition an alien civilization more advanced than us would be up on technologies outside of our understanding.

thanks to phys.org for the great info