While the argument that with the armory system not needing to unlock a class before you can use it is certainly valid, it doesn't necessarily mean it has to work that way. In FFXI there were 6 jobs that you could use at the beginning, it didn't matter what you started as you could switch between those 6 whenever you felt like it, then the advanced jobs were unlocked later.

And while it is not an exact correlation, I could easily see a situation where to become an arcanist you need to complete a quest or series of quests and then are rewarded with your first grimoire, and that is the only level one option available to you, which would pretty much close the class off until you actually did them.