Horn is one of those things so ordinary that I expect you have seen loads and not recognised them.
It could be that in your area all the horns were used up to make drinking horns or beakers?
Even before mass polling, as a material it had a certain rarity. (As a study of the Worshipful Company of Horners records shows).
And yes, they are a thermoplastic; heating up and moulding is how they are made. But I doubt soup cool enough to be eating and a quick wash wouldnt harm them.
These are new, polished spoons; an old Victorian hornware item would look dull and rather like wood. Unless you gave it a second inspection, you might mistake it for wood. (This is what I was searching for for that museum; though really only an expert like me would spot the difference between an old and a new spoon. But, if I had given them any new piece it would nag on my mind, wouldnt it?)