I would think the replanting story could be true, because I have had other things pop up in my manure pile, such as watermelon plants and bermuda grass (after feeding the horses those things). So I don't see why oats wouldn't do the same thing.
It makes me question why I am buying oats though, if many aren't digested.
I have heard that they ARE digested, and what you are seeing in the manure is just the hull, but I have actually taken a few oats out of the manure(!) and peeled them open, to see if it was just the hull or if the seed was still inside. The seed was still inside. So while the horses enjoy oats and probably chew up some of them, I also think many of them are wasted.
I am going to switch to barely when I run out of the current batch of oats. I HOPE barely is more digestible, because it is rolled and doesn't have all the hulls oats do. In my case, I want my horses to get some energy from the grain, because one is a lactating mare and she needs the calories.
I have heard oats are a "safer" grain to feed because they do have the hulls.