the undead were able to weild light as light is given to those with SELF BELIEF and they weilded it in life before they died so were already used to using it.
there are no undead pandas. -> there are no undead monks
if sylvanas were to "create" some undead monks the pands would probably leave the horde and stop being neutral.
and those "monks" in scarlet relate to the lights religious monks not the panda type who use Chi, as the scarlet ones use the light not Chi.