For those who don't remember, Chemist was originally going to be our first healer added back in HW. But early in development they couldn't get the Mix command to function as intended. From that scrapped concept they developed Astrologian, using cards as an alternative to Mix, and Machinist, using a gun as a weapon, since at that point a "gun-wielding job was already "hinted" at.

Additionally, for Geomancer, while the quests do draw parallels between them and Astros, it never outright calls them healers, they are referred to as "protectors". While a healer is a good conclusion, given their higher output and focus on "defense" them being tanks or tanky DPS makes more sense. And in the series they have a history of being tanky, so it's not foreign to them.