Is the internal ATI/NVIDIA format adding Green channel bytes and how many? Because it is the same Green/Alpha packing the only difference could be that they do something like DTXNormal = [4][8][4][Alpha].
Are you taking about the end of the tutorial where they mention ATI2/3DC? As posted above, this format is also known as BC5 - so you'll be able to find info on it under that name.
It doesn't store 4 channels, only 2!
So you get 8bit green & 8bit alpha, without wasting bits on red/blue like you do with the DXT5 trick.
The way this works with the shader is, the channels are called Luminace and Alpha (not green and alpha). When you sample from the texture, the shader receives float4(L,L,L,A) - Luminace is copied into R, G & B, which is handy because as mentioned in the tutorial, it's then compatible with he same shader code as "DXT5-swizzle" normal maps.