Title III is the set of regulations that implements the ADA. It's not exhaustive of what the ADA prescribes.
I'm going to disagree with this statement. Regulations are the implementation of law. Having worked in government for nearly 40 years, been involved in the development, implementation & enforcement of both, regulations are quite exhaustive.
Disney's lawyers will be quite familiar with the ADA and what, as a private company proving public access, it must do to meet the ADA. I'm also pretty sure Disney's internal counsel or an expert firm it keeps on retainer, reviewed both the original DAS and the revised version to make sure it complies with both statute and regulation.