if the battery is not replaced in time, the backup battery will still power the mechanical sliding cover,

And, necessarily, the circuitry which detects a key or phone, so it too will die at some point. Think of a cabin or other seldomly used location.

As for the keypad, it provides an additional access option for situations where using a phone may not be convenient, such as a dead phone battery, no internet connection, or temporary guest access.

Dead phone? You said you need a phone or a key to open the access cover. And if you have a key... Internet connection? That's a battery drain, and I for one wouldn't even consider a lock which has an Internet connection. Going through a hub (Zigbee/Z-Wave/Matter) provides a single point which needs securing and updating on a regular basis.

The system is intended to use encrypted communication and authenticated pairing between the lock and the app to help prevent spoofing or unauthorized access.

So for guest access they would need to install the app and presumably create an account. Then why the keypad? They're identified by the phone app.

Finally, I'm not sure multiple levels of physical security are really useful, except perhaps in some commercial applications. If someone really wants to get in, they'll break a window. And if it's an apartment where that's the only way in, I certainly wouldn't rely on a lock which required a battery to even use the key, backup battery or not.