Availability Zones vs Facilities

At the time of writing in April 2018, the most common number of AZs in a region is 3. The next most common number is 2.

The following from the S3 FAQ seems to be inconsistent with that:

 

Q:  What is an AWS Availability Zone (AZ)?

An AWS Availability Zone is an isolated location within an AWS Region. Within each AWS Region, S3 operates in a minimum of three AZs, each separated by miles to protect against local events like fires, floods, etc.

My assumption is that there is a hidden geographically isolated facility where required.

Sometimes the course notes do in fact use the word “facility” eg:

By default, data in Amazon S3 is stored redundantly across multiple facilities and multiple devices in each facility.