It sounds very out of line of what you'd expect.
At this point in time a new AI company would be bottle-necked by lack of NVIDIA GPUs. They are sold out for the medium term future.
So if Sam and Greg were to start a new AI company, even with billions of initial capital (very likely given their street cred) they would spend at a minimum several months just acquiring the hardware needed to compete with OpenAI.
With Microsoft they have the hardware from day one and unlimited capital.
At the same time their competitor, OpenAI, gets most of the money from Microsoft (a deal negotiated by Sam, BTW).
So Microsoft decided to compete with OpenAI.
This is the worst possible outcome for OpenAI: they loose talent, pretty much loose their main source of cash (not today but medium to long term) and get cash rich and GPU-rich competitor who's now their main customer.
They already do, though, has everyone forgot they got a Microsoft Research division?