Is this the same client that requires IE8 support suggesting that you use HTML5-only elements.. that aren't supported by IE8?
If it is the same client then they are commenting on a version of Adapt that is now quite out of date. If you need the best accessibility support from Adapt you should be using the latest version (currently v4.2.0)
Regardless, I would say that this is the client's opinion rather than an objective fact that the accordion component is not WCAG 2 AA compliant.
I would also draw their attention to this example on the WAI-ARIA best practices site showing an accordion that uses a button... possibly there's a case to be made there for adding an aria-controls attribute to the button to make explicit that it is linked to the <div> containing the text. I'll do some investigation on that front, but typically there are many of these new ARIA features that don't currently have any tangible impact on the behaviour of assistive technology so the value of including them is often questionable...