As I see in the code (this is very old code) the --x-offset is in longitude units (in degrees), the --y-offset is latitude (also in degrees), and there's also the --z-offset (in meters). I now updated the code and doc to reflect that.
To apply an ECEF translation, you can use the pc_align tool to create a point cloud from a DEM (or start with the input PC), with zero iterations, then apply a given ECEF translation transform, again with pc_align with 0 iterations, then use point2dem. To do shifts in projected units on a DEM, one can use gdalwarp (with cubic interpolation).