From the FAQ in that news post:
Q: How did the Flood manage to get to the Ark?
A: During the events of Halo 3, the Flood took control of High Charity, the planet-sized satellite and capital city of the Covenant. The Flood crashed High Charity on the Ark in an effort to thwart its ability to fire – and rebuild – the Halo rings. While most of the Flood threat was destroyed in the crash of High Charity and in the explosive end of Halo 3, even the smallest remaining spore of Flood can take over the galaxy, given enough time.

Hmm, but how would a spore have survived? I imagine there have been innumerable discussions relating to how the Flood specimens on (at least) Installations 04 and 05 survived the effect of the Halo Array. Whatever reason there is or isn't for that, I hope there is a satisfying explanation of how any Flood on the Ark survived.

But anyway; Woooo!, it's time for Flood time once more! And playing as the Banished should be cool. And Terminus Firefight sounds quite fun. Would have liked all of that to have been included in the HW2 Season Pass, but whaddya gonna do.

And, included in the Season Pass, Jerome and Ripa 'Moramee (So much ANGER) as leaders! And additional campaign missions! And maps for everybody! And more leaders!
I am excited.