I can't really say, for all i know, they could perhabs allready have coded/animated the system for using 2 kinds of magazines with the PPSh, they could even have modelled the PPD-40 allready, and it just hasen't gotten announced yet.
Or maybe none of that has been made, i don't know, and i can't really know how much time TWI spends on making a gun either, or how hard pressed for time their 3D guys are or aren't.
But considdering they found the time to make 2 guns that might only maybe have been there, i do think it's worthwhile to ensure that one of the most iconic guns of the conflict, and which is a standard issue in the game, is correct and works as it should.
And likewise, considdering the PPD-40 was issued in the first game, and a lot more PPD's where made than AVT's and MKb's, and that it would allmost certainly have been in Stalingrad (considdering it was still in circulation at the end of the war, long after they had stopped making them), i do feel it's a worthwhile inclusion aswell, especially since we can't include the PPS-43.
Making sure the PPD-40 is also a high-level gun, rather than a readilly avalible gun like it was in Ost, is also a very good way to ensure that we'll see more of the more common PPSh. In Ost, there was a tendancy for us to see too many PPD's and not enough PPSh'es (the recoil in Ost really contributed to that, mind).