Why OneKey cannot access what you type?
It is not technically possible for OneKey to access to what you type:
OneKey is a keyboard app, but you can't write or type with it. It doesn't have keys for letters & digits.
To type your messages and personal information, you keep on using the iOS keyboard.
OneKey gets active only when you need to schedule, use a template, or another functionality.
Why did Apple put that warning?
Apple is making sure you know what you are allowing for with that access.
But this warning is only relevant to traditional keyboards, that allow you to write & type.
OneKey is among the first apps to use the keyboard "space" to offer you more functionalities relevant to messaging, but that isn't really a keyboard.