That is correct, there is no such thing as a government-approved “certificate” proving compliance with Section 508. Here, from http://www.section508.gov/blog/common-misconception-section-508-certification:
“Section 508 certification doesn’t exist. Some people are claiming that they can provide vendors with a third party certification that their products are Section 508 compliant. Well that simply isn’t possible! No one, including the agency buying the product or service, can certify compliance or conformance, because there are no accepted tests. In the end the federal agency purchasing a product or service is responsible for determining if the deliverables meet their requirements.”
But that doesn’t mean you can’t ensure that your documents are compliant. Every AIM-remediated document is CERTIFIED ACCESSIBLE AND COMPLIANT WITH SECTION 508. We are dedicated specialists, making PDFs compliant is our only business. All remediated documents are accompanied by detailed accessibility reports from NetCentric CommonLook (accepted by HHS) and Adobe Acrobat. PDF/UA and WCAG 2.0 compliance reports are also available.