True, Admiral Marcus might never have been involved in S13 but I was just going with 'ease of storytelling.' The timeline from the Abrams-verse seems to be manipulated by something (Organians, God, Q, fate, destiny, Time Police) to being similar to the mainstream universe as much as possible.
Just in "broad strokes."
That's just my view on why TOS crew is together, at least. In-universe.