Aaron is no longer considered as cofounder by reddit. He fought for free speech.

Swartz founded Infogami, which merged with reddit when they were both early stage incubator startups. In a sense, he became a founder of Not A Bug (which became the parent company of both Infogami and Reddit), but Reddit the subsidiary and project and website predates his involvement. And Reddit, the project, was a big reason why Not A Bug was acquired by Conde Nast in 2006.

He provided big early contributions (migrating the code base from lisp to python was a significant project), but wasn't really a founder, and didn't really contribute much after that first year.