If Aaron Swartz isn't a cofounder of reddit, then Elon Musk isn't a cofounder of PayPal. [1]
Edit: As far as I can see, the situation is almost exactly the same. Elon Musk's X.com and PayPal merged into PayPal. And Aaron's Infogami and reddit merged into reddit. The only difference I see is that the reddit founders don't like Aaron being a "cofounder". If they were both mergers, then I think it's fair to call both Elon and Aaron cofounders.