If the "database" works like most other databases (eg. postgres or sqlite), deleting a row doesn't immediately cause the data to be wiped from disk, for performance reasons. Then as others mentioned you have filesystem/SSD logic that does something similar on top of that.

But you can do other things to mitigate this. For instance, give each app a set of rolling daily encryption keys, and encrypt new messages at rest. Remove the app, remove all keys. Nightly, remove the oldest key. Perhaps have the entire key database either stored in Secure Enclave, or if there isn't room, have the key database itself encrypted by a rotating single key in Secure Enclave. Now there's nothing that an attacker can do.