HI @Mauersics,
The overlay is not explicitly required, but we do need to have a way to validate achievements are working. Without knowing the details of the title, my guess is the team doing the testing did not see achievements working in the build they tested. Can you share the product ID of your title so I can look it up on our side?
Additionally, the demo would need to support achievements if the demo is available on other PC storefronts and supports achievements on those other storefronts.
Hopefully that helps.