It sounds very out of line of what you'd expect.
OpenAI continues to develop core AI offered over API. Microsoft builds the developer ecosystem around it -- that's Sam's expertise anyway. Microsoft has made a bunch of investment in the developer ecosystem in GitHub and that fits the theme. Assuming Sam sticks around.
Also, the way the tweet is worded (looking forward to working with OpenAI), seems like its a truce negotiated by Satya?