If I were Disney, I would have taken the chance and gone the way of requiring documentation. ADA law isn’t very well-settled, and line accommodations haven’t been tested in court. The Six Flags case was just recently filed.
Disney’s business model is all about people standing in lines waiting for things. Even if almost all DAS users were qualified under the ADA, Disney would have a good case that the strain on its system adversely affected other guests to the point of being untenable.
But it looks like they want to avoid the problematic documentation route and limit the people who qualify for DAS. At this point, I think all we can do is wait to see how well the system works.