You will not find many references to "Sowjetunion" or "die Sowjets" in contemporary German accounts... for them the enemy in the East was "Russland" or "der Russe"
But they also talked about being at war with "England" or "die Engländer" and that wasn't correct either, they were at war with the United Kingdom of Great Britain and Northern Ireland (not to mention the British Empire, as was).
To be fair, quite a lot of British people then (and some now) would say "England" when they meant the UK as well.