Some security must be enabled on your device to achieve the 100% secure when backing up e.g. a pin or password at least, fingerprint etc.
Copying down the 12 words, is like copying your banking pin on to a piece of paper and putting it in a safe place offline.
That is only step 1 in keeping the wallet secure.
Step 2 is to enable security on the device in order to get the 100% secure such as Fingerprint & Face ID.
The app will use the device’s security e.g. pin, password, fingerprint to build the private key to keep the wallet’s transactions secure.