Since (at least in my code) the SetSelectedUnit method calls SetSelectedAction (default to Move) before firing the OnSelectedUnitChanged event, adding it to the OnSelectedUnitChanged listener in the UnitActionSystemUI essentially just means that it is being fired twice. Maybe there is a case to be made here for why, but otherwise this seems redundant, no?
Yup after changing the logic to Select the default unit on start that does indeed become redundant.