What is the WCAG criterion for Heading order?

We are having a VPAT dispute with a vendor. The error we have asked to be corrected is "Heading elements are not in a sequentially-descending order." They noted that "per Accessibility team, this is a 'best practice' and good to have, not a legal requirement." I am trying to find the WCAG 2.0 A or AA criterion that requires sequential order. I can't find that it explicitly says this. Does anyone have some advice or a reference where I can prove that this is a requirement? Or are they correct?

Thank you for your time.