It makes more sense that summoners get it from a lore standpoint because Carbuncles themselves are offensive/defensive. I'm gonna guess the quest to unlock this will have something to do with the research the Arcanist Guild Master and what's her name with the pink hair were mentioned being up to after you finish the Arcanist quests.

I'm really looking forward to the update now. I love the Scholar faeries but the egis aren't much to look at. Garuda in particular is a bit of an eyesore and losing her contagion for a negligible DPS increase by changing to Ifrit hurts my MP and makes him hard to use in long fights.