This was no boast. The German army was largely decimated in the east. About 65-80% of the German armed forces were in the east, depending on when you do the count. The Germans had already suffered about 3.5 million casualties in the east before the U.S. landed in Normandy. Within 60 days after D-day, the Germans lost another 800,000 casualties on the eastern front. (Look up 'Operation Bagration'.)
"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.