If they were just shutting down the dedicated app and offering the same capabilities in the ChatGPT interface, I don't see why Disney would exit their deal?

Because Disney's deal was specifically and exclusively related to Sora, which was OpenAI's bizzare attempt at a TikTok like social networking site but using AI generated videos.

It was not a deal that allowed the use of Disney's characters for general purpose AI generated content using OpenAI tools.