Sony does own the IP. I remember as the credits rolled at the end its says something like "DS a Registered Trademark of Sony Interactive".
It was a surprise to me because I had thought Kojima wouldn't let another one of his creations out of his control. But that was the only way Sony would fund it. I have read that the game he really wants to make and is pouring his creative mind behind is the game that features his new company's mascot "Ludens".
I doubt we'll ever see a sequel to Death Stranding. I loved it but not enough gamers did.