Exactly, Secret Chats are stored only on one device. Backups and sync of any kind decrease security, because any of the end-points can be compromised. Supersecure communication should always be device-specific - ideally you should have a burner phone with a SIM card registered to a random person and use it only from locations you rarely go with your normal phone, and - of course - without logging into your real Google/Apple account.

Everything else is an illusion of security, because both iOS and Android have plenty of backdoors that can be used by some three letter agencies to hack into your phone and access your private data. This is the world we are living in, unfortunately.