According to the Facebook policy, article I.6 states:
Your website must offer an explicit "Log Out" option that also logs the user out of Facebook.
https://developers.facebook.com/policy
And as far as I can tell 'website' also means 'app'.
Also, the login implementation instructions says under "Policies" to include a log out button (https://developers.facebook.com/docs/facebook-login/checklist#providelogout)
Your app should offer an explicit "Log Out" option (See Platform Policies section I.6). Make sure that the logout button is easy to find and that it functions properly."
That said, according to the documentation for the Unity Facebook SDK's FB.Logout function: (https://developers.facebook.com/docs/unity/reference/current/FB.Logout/)
You almost certainly should not use this function, which is provided primarily for completeness. Having a logout control inside a game that executes a Facebook-wide logout will violate users' expectations. Instead, allow users to control their logged-in status on Facebook itself.
So, is a Log Out button required? If so, how is it supposed to be implemented without FB.Logout()?
Do not assume website stands for application.
If they did not provide an accurate definition you can't be held liable.
That's why you'll see a DEFINITIONS article in legal documents.
For your protection I'd make a copy of of each and send Facebook an email about it for clarification.
I am not a lawyer. This is general information. This is not legal advice.