No, Giant's might is not rage. You can still concentrate on a spell if you are Giantish. And I DO!

MC into warlock, Protection from Evil/Good, get large, and I have advantage and proficiency on CON saves to keep that spell up. So I can do toe to toe with elementals, fiends, etc and they all have disadvantage to attack me. (The ranged spell casters will still cause me some problems, though).