By the power of truth, I, while living, have conquered the universe.
Most likely outcome is that it's Truth's first glimpse at the Portal Artifact under the city - which the Covenant were seen glassing their way through to.
However, the structure doesn't resemble the Artifact at all, a friend and I came up with a mad theory that Truth had flash cloned himself (explaining the change in voice actor) and gone into a Forerunner Slipspace Pod to survive the Halos firing (as he knew what the Rings do, nee Contact Harvest).
Good times. :)