Many users, including myself, do not wake up from the gentle silent alarm on the Withings Steel HR. However it is still a great sleep tracker, and the Smart Wakeup feature is still very useful to using heart rate and motion to determine when to wake me up.
I would love it if the Smart Wakeup feature could use my phone alarm, which is on my nightstand. I typically use airplane mode while sleeping, but I would leave Bluetooth on just for this feature. The louder phone alarm does actually wake me up, and the wrist-worn Steel HR is a great way to detect a smart wakeup interval.
The Withings Health Mate app should be able to create alarms using the system alarm features (both Android & iOS). This is how all other sleep tracking apps are working (Sleep As Android, etc)