It turned out that the pumpkin that was refusing to go away was actually in the process of resurrecting itself. When I got curious this afternoon and lifted up the top of it, this is what I saw.
The seeds were all there. The close-up shows several of them sprouting.
So the about-to-turn-into-soil looks of the ex-pumpkin was only a front to hide the photosynthetic machinery that was being assembled underneath.
Photosynthesis cannot take place without sunlight. So I removed the top of the pumpkin and left the sprouts exposed. We will see how successful they will be in their mission to take over the backyard.
Updates may be posted.