Standards conformance (meeting WCAG) and genuine usability are related but not identical: a page can technically pass every WCAG success criterion while still being confusing or frustrating for real assistive technology users in practice. Watching an experienced screen reader user actually try to complete checkout, for example, often surfaces friction that a compliance-only audit would miss entirely.
This kind of testing is typically layered on top of, not instead of, a standards-based audit, since it captures real-world experience that a checklist alone cannot, though it usually cannot cover every possible disability or assistive technology combination in one round.