No it doesn't work for deworming. It can work on external parasites, as it may work by it's sharp edges slicing the parasites exoskeleton thus killing them; which is why it doesn't work internally, once wet the sharp edges are no longer sharp, so do nothing to the parasite.
It's best to deworm with products created for horses. If you've not dewormed your pony before I'd recommend getting a fecal done so that you can see what worms are present. There are paste wormers available for the type of deworming you need. What does your veterinarian recommend? If you want a general dewormer then an Ivermectin product might be where to start.
DE is not effective when it gets wet. I use it in my garden for roly polys; it must be renewed after watering or a rain. I'm sure there are lots of old remedies for things, and some may work. But parasites in horses may not be where you want to begin experimenting. Make an appointment for your new horse with the vet: floating teeth, fecal count for parasites, his recommendation for a good farrier, and ask about a source of good horse hay (cow hay is not suitable for horses).
I am with the others on the fecal tests. Not only do you want to use the correct dewormer for the type of worms, but one of my minis actually tested negative for worms several times and I don't like to treat her if it isn't necessary.