I think there's a bit of a semantics debate at play between us. Which happens. I'm just saying, in the example campaign I created above, your character knew about trolls throughout the previous week's table play, while you were fighting troglodytes. Before you ever encountered your first troll. In that moment, your character had stories of monsters (including trolls) in his head. He had the whole time you'd been playing him. Everyone around the table, including you the elf's player, just didn't know it.I hear what you're saying, but I think your subtle (and well-taken, and well argued!) point is missing the less subtle, and larger point.
The character did not, in fact, "know." You, the player, instructed the DM to make a check. The DM did so, and then imparted the knowledge to you. Once you (the player) had that knowledge, you (the player) created the backstory that would allow you to impart that knowledge to the character.
It seems like the argument being made by some, is that the DM tells the player what knowledge is in his head. Either by permission, or by dice roll. The player is never allowed to decide that for himself. I just don't dig that playstyle. Which isn't me saying it's badwrongfun. Just that it's not my bag, man.