"Secret chats are e2e-encrypted chats that never under any circumstances get backed up. Cloud chats are encrypted in the same way, but also have a built-in cloud backup. "
Of course he's not dumb enough to walk into the mine of saying it directly. But he justifies the need to split chats in the context of backups, which gives any reader an impression you can't do client-side encrypted cloud backups on secret chats. He never even tries to explain why he doesn't try to bridge the feature gap.
I'm fairly sure his claim was that it was easier to do it this way.