I believe a precondition for Sam and Greg returning to OpenAI is that the board gets restructured (decelerationists culled). That is probably good for MSFT.
Someone playing Game of Thrones is sneaking up with a dagger, but has no idea that MSFT has snipers on all the rooftops.
OpenAI is a non-profit, so, no material benefit to him (at face value, I don't believe this is the case, though).