“Microsoft has assured us that there are positions for all OpenAl employees at this new subsidiary should we choose to join.”
Microsoft can absorb all the employees and switch them into the new AI subsidiary which basically is an acqui-hire without buying out everyone else's shares and making a new DeepMind / OpenAI research division inside of the company.
So all along it was a long winded side-step into having a new AI division without all the regulatory headaches of a formal acquisition.