The purpose of a defined "single ended" output impedance isn't founded in IEEE 1596 as far as I'm aware of.
The measurement setup suggests, that the parameter of interest is common mode output impedance.
You can also read single ended output impedance as strict specification that defines differential impedance, too. (With a range of 80 to 280 ohm)
Drivers designed according to TIA/EIA LVDS specification have often current source outputs. They won't keep the strict interpretation of single ended output impedance and possibly the comon mode impedance specification neither.