By default the Slack iOS app uses an In-App Browser instead of Safari for opening links. This prevents the Standups app (or any app) from properly enabling deep-linking.

It's an easy fix though. Simply do the following:

  • From within a channel in the Slack iOS app, select the dots menu in the top-right corner
  • Select Settings from the menu
  • Select Advanced > Web Browser
  • Change to Safari

Now Standups links in Slack should properly deep-link to the Standups iOS app.