Gamist play works for a lot of people. Others like the game to involve more realism and sense, which requires reasons for things like that.
Only because you choose to be bothered by it, and to think things are unrealistic and make no sense.
Take the OP's story. Let's say you're at that table when it occurs, and since you know those FR novels backwards and forwards, your reaction to the player yelling out "She's a lich!" is to think, "Oh, great, way to break character, Elliot."
Then the DM explains, "No, Elliot's character was...(insert backstory that explains how he knows who Valindra is)".
Oh. So suddenly it turns out Elliot
was in character. You just used your own metagame knowledge to conclude he wasn't, which was a mistake. Sure, maybe
most people in Faerun don't know who Valindra is, but certainly some do. It's actually entirely plausible that a PC would recognize the name.
But in this case, as we know, it really was OOC knowledge, because he told us. Right? Just the fact that it bothers you for that reason means you are (wait for it) metagaming: you're using your out-of-game knowledge to decide that the character's action was unrealistic, even though we just determined that it could just as easily have been completely explicable and, thus, realistic. If you were
in-character, your reaction would have been, "Are you 100% sure? How can we verify that? What should we do? GAH we're just lowly adventurers she is going to STEAL OUR SOULS!!!!" But, no, you were in the metagame, worrying about whether that knowledge came from a legitimate source.
You dirty metagamer you.
When I'm playing D&D and somebody acts on some knowledge that I don't know, or maybe that I do know but don't think my character would know, or even that is presented in a way that I know they only know because they've memorized the Monster Manual...honestly I barely even notice. I'm thinking, "Cool, he knows some interesting naughty word" and I'm not really distinguishing between the player and the character, and I don't really care where the info came from. So, no, there's zero impact on my sense of realism.