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.)