Yeah, I'm not sure what consists of a win or loss in Flood mode.
I have played 6 Flood games and have won all six of them (win rate is 100%), even though there were some games there where I clearly started as a Spartan but became a Flood by the end of that round.
I would suppose you would "lose" if you started as a Flood, and failed to convert all of the Spartans before the time runs out. But I've never been in that situation before, so again, I do not know how wins/losses work in Flood.
IRON MAIDEN IS MY RELIGION! Gamertag: Demigod Elessar