Never used before doesn't mean doesn't exist in the campaign world. Unless it's never been used in the world before, there's probably tales of it somewhere.
PCs can't invent gunpowder in my games because the laws of reality are different -- whatever IRL formulation the player might know doesn't work in the game world due to different physical and metaphysical laws. But, if a player wants their PC to set up a project to invent it, sure, sounds good to me.
Just like if a player wants to think they know things about a monster, that's fine with me. They may not be right.
And here's where we digress. I, personally, know many rare pieces of knowledge about my world, IRL. A lot most people don't know. And I learned a lot of it from other people telling me about it. So, yeah, I'm not sure at all that potential rarity makes it so that a PC can't know something. This is more GM reasoning, and still makes metagaming entirely the GM's problem.