Thanks for the clarification! Now I'm thinking that maybe Snape did invent Sectumsempra, but Lupin and the Marauders didn't realize that he did. After all, we don't see any of the kids at Hogwarts, even Hermione, "the brightest witch of her age," creating their own spells, so Lupin may have just assumed that Snape found it in a book or something. And other people might have started using the spell after watching Snape cast it, further muddying the waters.