Hi Scott
Sorry about the issue. This is a limitation currently.
As you may have seem with some of the Cascade stories that our team creates, such as "Justice Deferred", it is possible to provide that alternative version for mobile but it requires downloading and self-hosting the app. This approach is described in this blog post:
https://community.esri.com/community/gis/web-gis/storymaps/blog/2017/02/24/story-maps-developers-cor...
It's really a developer solution, so it is on our Story Maps Developer's Corner blog rather than on the ArcGIS blog.
An alternative is to use a static image instead of a video on the cover page. Some authors choose to do that but others choose not to do that because they really like desktop users to see their video.
Rupert
