That statement is correct. E2E encryption is only in effect between two devices communicating with each other. The moment the messages leave the app in any capacity (e.g., screenshots, backup, forwarded to another convo/contact, etc.) it is not longer protected by E2E encryption.
The backup file itself that is uploaded to iCloud/GDrive is encrypted; see: https://security.stackexchange.com/questions/136072/how-can-...