My issue is, what type of "pumpkin" are we talking about??
There are a LOT of different things called pumpkins. Not all are the same. There is a difference between carving pumpkins, baking or pie pumpkins, giant pumpkins, strictly ornamental ones...
I assume that the ones being used here are carving pumpkins, but, who's to say that it's not a different type who's seed works?
Actually, it seems more likely to me that it would be the pie pumpkins that would be where this theory got it's start as those tend to be heirloom varietys and therefore are older and more likely to be the ones that inspired the old wives tale (true or not, I'm not judging that). Plus, I find it unlikely that a homesteader would spend the time, energy, and space on a huge plant that makes greatly substandard food and it's only real claim to fame is making a decoration that rots on your porch. It would be much more likely (in my mind anyway) that a housewife making a pumpkin pie or otherwise preparing a pumpkin for cooking, would scoop the seeds and give some to the chickens in the yard.
Maybe, someone had a bag of pumpkin seeds set aside for planting in the spring and the chickens broke in and ate a ton of them, then they expelled worms to an extent that would have been noticed.
Something started this rumor or theory (whatever you want to call it), most things like this don't come completely out of the blue, but, I seriously doubt that the event that caused this information to be passed down was caused by a carving pumpkin.