Sony owns a copyright over the actual, finished product on the disc (or digital storefront) for the purposes of protecting the sale of said product, but they don't own the actual content in any meaningful way beyond the actual source code. It's produced under a license and as you've said, all the actual IP (names, characters, locations, gameplay, concepts, visuals, audio, etc) are all Activision's property. Considering the remake would likely not use any original source code, Activision can do whatever they want with it because it won't be using the one piece Sony actually owns.