In Aaron Swartz's own words from May 2007
> One of the points of the merger was that we would all call ourselves co-founders, so that's what I've been doing. I'd be happy to stop if that's what Steve and Alexis wanted, though.
https://www.reddit.com/r/reddit.com/comments/1octb/reddit_co...
This comment by spez is also interesting https://www.reddit.com/r/todayilearned/comments/d2njs/til_th...
It seems AS was given the title of co-founder by contract, but perhaps, the language was vague enough or they didn't fear retaliation so Steve and Alexis decided they didn't want to acknowledge him as co-founder.
Edit: here are some more details: https://observer.com/2011/07/rumors-acquisitions-did-reddit-... including comments by PG.