>The group’s other founding members are world-class research engineers and scientists: Trevor Blackwell, Vicki Cheung, Andrej Karpathy, Durk Kingma, John Schulman, Pamela Vagata, and Wojciech Zaremba.

That sentence explicitly lists who the founders are. Note that neither Musk nor Altman are listed before the period closes it out. The next sentence goes on to say...

>Pieter Abbeel, Yoshua Bengio, Alan Kay, Sergey Levine, and Vishal Sikka are advisors to the group.

Alright, cool - we've moved on from the list of founders, I see. It's not until the next sentence that we find out that..

>OpenAI’s co-chairs are Sam Altman and Elon Musk.

So, OpenAI themselves made it a point to separate "founders" from "co-chairs" in their own announcement. I don't really know how it could be any clearer than that.