What's a Pumpkin Pot?

I saw this picture on the Internet with very little explanation.  The caption said - "After cleaning out the pumpkin to make your holiday pie, fill the empty cavity with potting soil/dirt and add back in some of the seeds.  Water and watch the plant grow."   

I think this is a super idea but I just don't know how it will work.  Will the shell rot before you get a plant?  Can I plant this entire pot in the ground when it starts to get soft, prior to the mold and stintch?  Or - is it possible to put a coating of some sort on the emptied out pumpkin in order to preserve it for a while?

If anyone knows any info on this, I would love to hear it.