Resike that has to work. Otherwise you found a bug.
*edit* Tried it. Works both ways.
/run print(UnitIsUnit("nameplate3","target")) --returns true
/run print(UnitIsUnit("target","nameplate3")) --returns true
/run print(C_NamePlate.GetNamePlateForUnit("nameplate3"):GetName()) --returns NamePlate3
/run print(C_NamePlate.GetNamePlateForUnit("target"):GetName()) --returns NamePlate3
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