Relentless states it increases damage by x% but does not say to what damage or weapon type, so I assume it means all.
In testing it with hammer throw, relentless does not increase hammer throws damage. Tested with 6 and 7 points in Relentless and 0 points in Relentless
This was tested by attacking the target dummy and seeing the same dps numbers pop up, and attacking mobs in a zone and watching them die at the same kill rate. Either wording is wrong on Relentless or Hammer Throw is missing a damage modifier tag.