Comment by ragona
I don't think that's quite right. PQ attacks focus on the "trapdoor" functions in asymmetric cryptography, _not_ the symmetric encryption that happens after key negotiation. The current concern is that a future attacker could unwrap the symmetric key, not directly attack the symmetric encryption that is used for something like backups.
(Note: I didn't actually dig into the backup implementation, but my guess is that it's more of a KDF -> symmetric design, rather than the sorts of asymmetric negotiation you'd find in multi-party messaging.)
If the app takes your disappearing message, encrypts it with a static key that never changes and is never deleted, and uploads it to the cloud, then the message is never truly "disappearing." A "post compromise" event will allow the attacker to decrypt that ciphertext at any point in the future. All of this ratcheting is undone by backups.