I don't know if that was lore actually, but we are talking about the heroes, which they encluded on the game lore so whatever, would the marine think to sacrifice to save the world if there's no other way?
imagine, the marine from PB1 and PB2, had a lot of armor and gun upgrades, a lot of guns, passed a lot of missions, and then you see him dies on PB2.5, marine dies, proxy cries, noir lime salute too.