Thats exactly what I think.Best thing they could do with this is reward the people who have bought the main SR4 game and let them use the content from Gat out of Hell in the main SR4 game!
You are missing the part where it says "stand alone". There are 2 ways they can do that, a complete and separate install of the new game or as a module that's installed into the main SR4 folder, uses assets from SR4, but is accessed alone.
My educated assumption (I could very well be wrong) is that either way, you wont be accessing any new content from your SR4 game save, it will be completely separated.
GooH is supposed to be completely standalone meaning that you don't even need SR4 to play it.
If it is like that you won't get access to New Hades, ... - in IV.