it sounds a little similar to the deal with sony; in that they still have all the rights to spiderman (aside from sony not getting anything from disney using him) so disney going ahead and making a hulk film would mean losing money?
Nah, the Sony situation is that Sony still has full rights to the character and movies and all that, but voluntarily brought in Marvel Studios/Disney as a collaborator. Disney didn't get any rights back in that case. Even though the new Spidey film will be in the MCU, it's still being made, produced, and distributed by Sony
Universal, as far as I know, doesn't have the rights to Hulk as a character (as in, they can't make their own Hulk movie) but still have distribution rights on any Hulk solo films. So unless Disney buys those, which would be stupidly expensive, they either have to give up that distribution cash to Universal or just not make a movie. They're opting for the later
(For the record, this is why The Incredible Hulk is in a weird place. It's part of the MCU, but not on Disney Movies Anywhere and is still distributed by Universal to this day. So if you buy TIH on home video, Universal gets the cash. Same situation with A New Hope and Fox, funnily enough)