Well, you see, Warhammer Fantasy isn't at its core a roleplaying game. You don't show up with armies made up of unknown points, so that sometimes it is 1,000 vs 2,750. Anyway, where I play at, you may look over each other's lists before the start of the game in pick-up matches (Because we all make mistakes now and then). I even specify which hero has what magic items so that no one claims that I'm teleporting them around (you don't actually have to say what magic items a character is carrying till they are used). Now, in tournaments, where army lists are reviewed by non-players before the start, yes, you wouldn't know that he happens to have an assassin till he reveals it.
But again, DE assassins at least have in-built protection from it. If the others do not (unlikely), and you know that the list has that kind of character, and you aren't playing some super-serious RP campaign, then there isn't any reason why the statuette can't target them.
Heck, to make it not meta-gaming, a skink priest read it in the stars that there'd be an assassin there, which is perfectly in line with an army preparing for battle with seers available.
EDIT: To make the meta-gaming thing more apparent, unlike DnD, when your General dies, you don't automatically lose. You aren't playing a character, or even from a set perspective. You making moves is an abstraction of the entire military network of your army and its logistics doing things.