Sort of. The story is that he had a stroke, and no one was willing to call a doctor. He wasn't dead, but he was unresponsive and incontinent, but he had also just kicked off "The Doctor's Plot" and his previous physician was being tortured in the Lubyanka at the time. So... who wants to volunteer to help him in that situation? It was also a power structure that encouraged people to always defer to higher authority, but Uncle Joe was the highest authority.

A possibly more realistic claim is that Beria poisoned him, or at least took the opportunity to keep anyone from helping him for 8-12 hours after his stroke. When people finally did attend to him, the state of Soviet medicine was so poor they applied vinegar and enemas as a treatment.

So yeah... dead Joe.