Story Ads are fullscreen ad placements that appear in AMP stories. Ad placement & insertion are orchestrated by the runtime so the story ads are shown to the user only once they have fully loaded. As a result, Google says story ads by definition are 100% viewable.
Vamsee Jasti, Product Manager at Google, says “story ads use the entire screen to convey a brand or message using a combination of video, image, or animations.” Users continue “the tap gesture to skip over the ad if uninterested but also has a consistent way to explore the ad using the call to action button. In addition, every ad has a ‘consistent’ ad attribution label, so users are easily able to distinguish between an ad vs organic content inside a story.”