Not sure where Relic got that from. I did some research and I could not find anything about Shock Armies getting more SMGs than others. All I could find was that Shock Armies got more tanks and artillery and generally more infantry divisions than other armies so that they simply had the numbers and support to punch through more easily.
The new Soviet Shock Army line-up represents that.
I admit I also came up dry while trying to find it as well, to prove my own point. Awhile ago one of the Community Managers said they had a stack of books they sourced a lot of their stuff from, including bulletin descriptions, but I have no idea what books they might have used.
The closest source I found was that all Soviet armies had a fully submachine gun equipped Assault Company, not just the Shock Armies. But my dream for stock assault squads is even further out of reach despite the historical basis. I also found that Shock Armies had a larger concentration of Guards units, which is irrelevant, but still interesting. Alas the most I have to fall back on is that using PPSh Conscripts and Shock Troops together is just really really fun, and it's a shame to see it go.