That page also tells you why in the “Causes” section, right above what you quoted.
“This issue occurs if Facebook integration isn't available for your organization. Validation rules block access to features that don't apply to certain organizations. Even though you can't disable the feature, you don't have to be concerned about the feature being used by people in your organization. If the Facebook contact sync feature isn't available for your organization, this means that the feature is blocked at a deeper level.”
I’m not sure how you could’ve missed it, quite honestly.