Correct, for tax purposes corporate losses remain with the corporation. Microsoft and the other owners don't get the benefit of OpenAI's losses. At best, they get to write off their investment in OpenAI if the company dissolves, at which point their maximum tax write-off is their capital investment.

Note: other people seem to be confused because companies can write off investments in corporate subsidiaries before the subsidiary is dissolved or sold...for book purposes. This creates what is known in the accounting world as a book-tax difference. If you have a few weeks to spare, look up tax provisions...