Most people use 1-shot to mean it will instantly kill a person, regardless of HP, regardless of class, regardless of buffs on. I think that's where the confusion is coming from.
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They are part of the Mainstory if a mainstory quest (those with the meteor icon) tell you to kill them. For now they (Garuda hard, Ifrit hard and Titan hard) are only required to be cleared, but not part of the mainstory itself.
Only at Moogle King the primal hardmodes begin to be also the primal storymodes.
Only after reading this thread do I realize how lucky I got. I one shot this in my first try last night. I was healing and it was super easy for me, I spent most of the fight in CS. I didn't realize how much coordination was involved, I'm thinking I joined a preformed group of 7 because no one spoke of any strategy and they all cheered when they saw they were getting a bonus since I was new.