They did rebuild the game from scratch. The Terraria we used to play on the Xbox no longer exists. It's gone forever, except on old-gen consoles. Why can't people realize that they re-coded the entire engine of the game?
I used to do programming several years ago. It definitely is not a simple cut and paste. At best all you'll get for free is the sprites and the data (attack, defense, hp, map size, etc).
Everything else is literally a clean slate fresh from scratch.
I have a lot of respect for Pipeworks as well as Engine for doing console builds.