I think I misunderstood. It seems like Heroes of Stalingrad is a full game. But there's an expansion called Rising Storm that has a lot of content, but this only contains a demo of it.
You still get the full Heroes of Stalingrad experience, and the rifle content of Rising Storm is a bonus. But the store page is really confusing because they have all the details and sales points of the Game of the Year Edition even though it's not the GoTY edition they're selling on that store page.
All of the above is just speculation because it's really confusing. I'm still not sure if any of this is right.