I find it especially doubtful that Army Group North should be so easily pushed back in this game when it has Leningrad nearby as a major port to draw supply from. This, indeed, is a very good reason for taking Leningrad in the first place. Aren't Helsinki and Leningrad ice free during winter?
The Gulf of Finland freezes over on most years, with a layer of ice of some 8 to 16 inches on average. Both in the Winter War and the 1941-1942 winter, there was a significant amount of ice on the Gulf of Finland. Helsinki could be reached by ice breakers with limited difficulty. Leningrad? Not so much. It required and requires lots of ice breakers to stay operational in a severe winter. In 2003, ships were locked in port for a long time.
The Soviet Union didn't have any ice free Baltic ports. Currently, due to the Soviets capturing Koenigsberg and Russia keeping control of the city, Kaliningrad is the only Russian ice free port in the Baltic Sea.