It's not meta-game - it does correspond to real knowledge that the characters possess - but it's... kind of abstract and hard to deal with. There's a lot of twisting to make it fit into a system that's roughly analogous to the original meaning of the term, while still being simple enough for players to understand it.
Part of the deal with class-and-level systems is that they bundle a lot of different abilities together, and then allow experience with any one of those things to advance the progression of all of them. Within the narrative of the game world, fighters get better at fighting by fighting, and wizards are supposed to be digging up arcane lore in all of these places, while rogues are using their rogue skills all along. In AD&D, they tried giving out experience points based on those priorities - everyone got XP for fighting, but fighters got more from fighting, wizards also got XP from casting spells, and thieves also got XP from finding loot or using their skills.
Maybe, back in the old days, experience was purely meta-game. I can't speak for anything before 2E. I've never seen a game where advancement was purely meta-game, though.