The Marvel Studios didn't belong to Disney at that time and so the deals were postponed, e.g. Iron Man was distributed by Paramount and there was a deal, which didn't become public any more and it worked smoothly and without problems between the studios.
The Marvel Studios probably won't release any Hulk solo film unless they finally agree with Universal or squeeze the money.
The same is true for the character Namor. For those of you who don't know him, Namor is the Aquaman of the Marvel Universe. He was invented before Aquaman, but Aquaman is better known. Namor might also not get his own solo film unless Universal would give the OK and take over the distribution and so Disney would have to pay money again, but they probably don't want that.
But Namor could still appear at the MCU and there were already some easter eggs in the past MCU movies that might point to him and there are rumors that he will appear in the upcoming Black Panther 2 or Shang-Chi.
Marvel is allowed to use this character but again not with a solo film, because they only own the production rights. It's a bit difficult with a solo film. You could produce a solo film but then Disney would have to pay money to Universal and Disney doesn't want that. So we have the same problem here as with Hulk.
Marvel was just on the verge of bankruptcy back then, had to knock out all his film rights somehow so that they could stay alive and so today a Kevin Feige has to bite his hand more often. But bit by bit the rights ones come back somehow.
Often for example, if a film or a comic character wasn't used again by the right owners for a certain time, then the rights often fall back to the Marvel Studios. However, this is very different with the respective studio contracts. Maybe we will see Hulk in a solo movie sometime. There are a lot of characters where the license situation is a bit undecided, because some characters have a very strong occur in the different comics and therefore Marvel has to check very carefully if they can use the character and how to use it.
So you notice that it's still a bit confusing with the right ones, DC is a bit tidier and better organized, but what the hell, that's sometimes the case with the rights and licenses. I hope I could explain that to you a little bit.