And it opens a separate chat from the existing one.
I don’t see the problem. The secret one has the lock icon to clearly mark it. There’s no way one would accidentally pick the wrong chat. Delete the old, unencrypted one to be sure.
It’s obscure enough that you could say the UX deprioritizes (which at best is not an actively malicious design choice) usage of end-to-end encryption.
I agreed in another comment that there should be an “encrypted by default” option somewhere. I’m not claiming that it’s perfect but the claim in the blog that it’s super complicated is just not true. At least calls are P2P-encrypted by default.