Ambient Mode and App Standby are not the same, ambient mode allows your app to keep running on the foreground while staying visible to the user, and if your app is running on the foreground it will not enter to the stand by mode, see next paragraph:
App Standby allows the system to determine that an app is idle when the user is not actively using it. The system makes this determination when the user does not touch the app for a certain period of time and none of the following conditions applies:
- The app has a process currently in the foreground (either as an activity or foreground service, or in use by another activity or
foreground service).
As you can see the ambient mode allows you to stay on the foreground thus not allowing to enter Stand By, it's worth noting that to be able to use the Ambient Mode the user has to enable the option Always on screen on the device.