Well, a 2d sprite is simply a plain, with 4 vertices, and usually (without messing with uvs) that plain has UV coords like this:
_________________________________________
* = vertex
_ = index
. = empty
0,0......1,0
*______*
|........./...|
|......./.....|
|...../.......|
*______*
0,1......1,1
_________________________________________
and thus, this plain uses the whole image, since it's UVs are covering 0,0 to 1,1 in the image, modifying the UVs will change the results, as you saw in the Dark Invaders tutorial
the animated sprite would too big, as it will hold over 10 backdrops, which is actually too large, heavy for loading, using UVs is way way faster and more efficient