Well, the first Halo 5 trailer certainly looked like it could be the Ark. However, the bookend cutscenes of H2A show that the Arbiter's Lich has a hologram of the Forerunner Warbird. Implying they're going to the same place Chief was at. And when they arrive, the Lich's pilot shouts out, "Covenant forces inbound! Glory to the Arbiter!" So it sounds like they're at a location that isn't as hard to reach as the Ark. Plus, the only way to get to the Ark, is through the portal at Voi.