To be fair, Aaron wasn't really heavily involved with Reddit. He was involved with Infogami, which failed and was merged with Reddit. Per this post, he got equity in Reddit as a result, but only worked for a few months on the actual site.
After his death and the media portrayal of him as a martyr for free speech, Reddit started claiming him as "co-founder" much more vocally than they had previously. While technically he had that title, his involvement on Reddit was neither starting the company nor working on it for any extended period of time.
That being said, given what Reddit's become, wherever his spirit roams now, I'm sure he's relieved to have his name off the site.