I was able to test it like this:
- I published a closed alpha version of an app - open only to my email (beta should work just the same)
- Opted in via the opt-in link in the developer console (it's on the APK section)
After that, clicking the dynamic link I created on the Firebase console worked as expected, without having my app out in the open.
Caveats:
- Connected Google users - might cause a problem if only one is on the alpha group list
- It takes a bit of time to get started. Make sure you can see the app in Google Play before trying to click the link
Updates, fixes and changes take time until they actually appear on Google Play. Google Play now offers internal testing, a level below alpha that propagates updates quicker.