The battle in the Prologue can be taken as purely symbolic, since there's absolutely nothing that grounds it on any canonical basis. We don't know what planet it is, what year it takes lace, and so on. What you see is the overwhelming might of the Covenant crushing humanity and then seeing the Spartan-IIs bringing renewed hope as they emerge out of the smoke, John leading the fray as Halsey states that her Spartans are the future of humanity.