You haven't really understood. Ivana is not a spy. Or maybe she is. Just like the is no canon explanation for where Azalin is, there is no canon stat block for Ivana (or any other dark lord). "Similar to a spy" is just suggesting where the DM should start if the story they are telling puts the PCs in a position where they might attack that character. The DM can give her any abilities they like, such as Immunity To Charm or Legendary Resistance. Or simply rule that the charm fails and she pretends to be charmed in order to trick the PCs.