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 mobile app.