The decoy passcode feature should boot into a separate partition that looks like a normal phone setup, and during that time quietly erase the user's actual data. They would never have known if it worked like this.
[1] https://veracrypt.io/en/Wear-Leveling.html
https://veracrypt.io/en/Trim%20Operation.html
[2] https://nitter.net/GrapheneOS/status/2082153517234676150#m