You'll find this new feature in Journals > Settings > Passcode. The passcode does not sync between devices, so you can use different ones for each device.
Once activated, you can also use Face/Touch ID - useful if you forget the passcode, otherwise, if you are certain that your data is synced, you would need to delete the app from the device and re-install it.
You can also select the time to automatically lock Hibi Journal once you leave the app.
Passcode feature is part of Essential upgrade. As a reminder, if you already purchased this upgrade, you get this new feature for free! And if you haven't, you now have one more reason to do so!
Next update will bring a rich text editor and French, Japanese, and Portuguese languages.