-Disney, while in charge of Marvel, is contractually obligated to act reasonably in regards to Universal's proposals for new attractions based on Marvel's characters.
-Universal presents a redo of Superhero Island to Marvel brass, who give their stamp of approval. Possibly due to Universal agreeing to drop all Fantastic Four and X-Men references from the redone land to stick it to Fox.
-Disney has no desire to allow this to go down, but under the wording of the contract has no real way of stopping it, so they approve it.