Why not create the details on top of the existing face?
https://cgcookie.com/lesson/adding-the-final-details?wtime=191
Like Adrian mentioned, it just seemed easier. You could definitely have cut in the same features on the existing face using the knife tool, but it would have been harder to get perfect triangles. If you were to use the same approach of adding new primitives but on top of the existing face, you would end up with overlapping geometry and still have to delete the face eventually anyway.