3D objects should always maintain it's workspace in the layers panel unless you combined/flattened the layers. You should be able to edit the "texture" applied to the 3D object. The texture "decal.jpg" used in the tutorial is 2D. So the answer to your question, if I'm understanding it, is yes...............you should have no problem replacing the texture in the 3D object with a 2D image.
Sam' s answer is correct.
Textures for 3D work in Photoshop are always 2D jpeg.images.
Only the values you put in the boxes of the "Texture Properties" do the wrap-around effect on the balloon.
In case you want to change the provided image and replace it with your own jpeg.image, this is the place to do it.
But you might have to adjust the values in the boxes.