"This was no boast. The German army was largely decimated in the east."
I believe this is an issue of semantics, but it was, indeed, a boast, and you supported why Stalin could thump his chest before an awestruck British and American contingent, and why he could make demands to them.
"Within 60 days after D-day, the Germans lost another 800,000 casualties on the eastern front."
Okee dokee. We all agree that the Germans got hammered by the Soviets. Aside from his boasts, Stalin went into propaganda mode to protect his image. You'll agree that this is an easy thing to do once you start winning. Remember, the Soviets were on the brink of collapse themselves with the Germans knocking at the door of Moscow, and Stalin, himself, thought he'd be arrested for incompetence.
I have to wonder how much hindsight is involved in the memories of the German combatants. I've read a number of autobiographies of German officers who admit that by late 43-early 44 they knew they could not win the war, could barely hope to survive. Many of them claim that their motiviation was the hope that, in time, the Western allies would see Stalin and the Soviets as the real enemy and would join with them. Of course, that is what did happen once Hitler and Roosevelt were out of the picture.
Another common claim is that "unconditional surrender" was a mistake, that it encouraged even those who were sour on Naziism to keep up the fight to save the fatherland.