The point is that it is not in the Platform Policy anymore, so Xenforo forum administrator as a Facebook developer do not have obligation to force logging user out of the Facebook at all even it is still "recommended" or "best practice." At least please consider making it a system option and let us decide.
Because we are serving our users, not Facebook. The practice is not technical recommendation or best practice, Facebook stated it clearly that they want the behaviour to be "as users expected", which in the reality is actually the other way round. Of course I have no other choice but fix it by myself if XenForo chose to keep it like that, I will just be disappointed.
If XenForo doesn't log you out of Facebook when you log out of the forums, after the logout redirect you will be logged in again immediately because the script detects the Facebook session and trigger the XenForo login routine.