That doesn't mean you will be restricted to leveling up only one class though, it merely states you can only master one, in my eyes that means I can only obtain one Job title for DoH (ie: Blacksmith → Airship Hull Builder). I'd still be able to level all my DoH classes to 50.

That is my speculation, as once again I will repeat that Square-Enix themselves stated they would not force/mandate the restriction of players who had something already leveled, which means its IMPOSSIBLE (and very unfair for new players) for them to only let you level up one class, and yes that statement was made with Yoshi-P's team, not Tanaka. I believe it was made right after that massive poll they did in ?December?, somewhere around that time.