Jobs are not bound by lore. Jobs can, and will, change as needed overtime and their lore (including their specific quests) will be edited to reflect this. This has already happened a few times, most notably with Summoner being the most recent subjected to this.

For example: Arcanist, Scholar & Summoner didn't exist until 2014 with the release of A Realm Reborn. Prior to that Thaumaturge was the "poison mage" class and very much is not anymore. Conjurer was closer to the ARR+ Thaumaturge back then. In fact, many of the abilities and spells that we had back during those times are still present in the game and still utilized by enemies we face.

Thancred, prior to Rogue & Ninja's release in ARR, was actually a Gladiator. He didn't make that shift until it was available.