This I didn't know! Why would he want to work with MS at this point (given this information)?
From what I know Kojima productions had a bunch of projects in the works. But one of them is kind of a experiment, similar to his GBA Boktai 2 solar Boy. He wants to do something cutting edge in terms of interactivity and delivery. Knowing MS's reach and MS's investment in cloud tech he wants to do a project that utilized that system.
Sony was uninterested from what I hear. But also in the world of Game studios, going to another publisher ups to anti for previous companies you are close with to get sweaty feet and come to an agreement or new contract that sweetens the pot.
It's a tactic used for getting more money for your studio, or bargaining chip that you will do business elsewhere. I don't know if its been negotiated to have Sony buy Kojima Productions, I believe it was talked about. But Kojima like shis independence, so he could still be working on MS's project while also taking a contract with SOny.
Bluepoint did this with Titanfall and other remasters for other companies while working with Sony. If your independent you can have multiple contracts. Like Platinum Games who has too many IMHO.
But he will make a announcement about his involvement with Sony on "some" projects. or He could be announcing his merging with Sony.