Hmm, according to the Eorzea Database on the Lodestone it is indeed a reward for completing the final ARR 2.0 main story quest 'The Ultimate Weapon' (meaning it was a recent change that would have been added when Stormblood launched).

If it isn't in your inventory, then try the Calamity Salvager and see if it's included there as your inventory may have been full when you completed the quest, and thus were unable to receive the reward.