No solution so far, unfortunately.  It would be nice to get a response from a member of the Shopify team on whether I'm just missing something about how this process is supposed to work, or correcting their documentation to remove the suggestion that...

I'm following the Shopify Tutorial to add a checkout extension post-purchase survey to thank-you and order-status pages.  The tutorial is based on an extension-only app (no external app server).  Within the tutorial, in this section, the text states ...