Yeah, and based on my very quick look through of their 990 their status looks correct, and their mission as summarized on page 1 doesn’t directly contradict working with Microsoft:
OpenAIs goal is to advance digital intelligence in the way that is most likely to benefit humanity as a whole, unconstrained by a need to generate financial return. We think that artificial intelligence technology will help shape the 21st century, and we want to help the world build safe AI technology and ensure that AI’s benefits are as widely and evenly distributed as possible.
None of that prohibits them from working exclusively with MS or making a profit. It’s a vague enough “public good” that it probably satisfies their 501(c)3 status.
Unfortunately there are a lot of dodgy nonprofits, big and small. I just can’t tell here if OpenAI is one of them.