I'm six months late, but here it goes:
Yes, it is the same thing, although Fisher's definition is somewhat difficult to grasp. He defines the 'Local Offset Viewshed' as following:
If the target point is in-view, then the vertical offset (the vertical height) between the land surface at the target location
and the line-of-sight to the next local or global horizon in the
direction of the line-of-sight is reported as a positive number.
If the target is out-of-sight, then the offset is reported as
a negative number which is the height between the land surface at the target location and the line-of-sight to the previous horizon in the direction of the line-of-sight.
Fisher (1996, 1298)
His measure is thus based on the "land surface". However, when dealing with a concrete problem he states:
The local offset shows the height the new construction can be at any location before it pierces the horizon from the viewing point and so the maximum height of the structure at any location to minimize visual impact. (idem, 1300)
If the building is considered as the target, then the measurement is from the target's surface (?). This is exactly what the 'invisibility' option in the QGIS viewshed plugin does (the vertical difference between the target's top and the line-of-sight touching the nearest horizon). If target is 0, then the difference is from the bare terrain.
PS, I'm the author of the QGIS viewshed plugin.
