What ability is this, and what class uses it?

[Abriael]

Now, one might argue the weapons by their sides don't denote what they are since they're NPCs, but that wouldn't make sense. Why? There's already tons of arcanist/musketeer NPCs with their weapons, so why not give the quest NPCs the proper weapons?

Which brings us back to the main point. Whether that NPC was or was not a ninja (marginal), there are NPCs in the game that belong to classes that at the moment aren't available to the player. For what we know they could even belong to a class available to the player, but be higher than the current level cap.
Or even more probably that ability has just be added for scenic effect, which is the whole point behind a cutscene. Showing something that can't be shown during normal gameplay.
I would say that there's nothing wrong with it.