In the lesson, the authors describe 7 S3 options. For each (except S3 Outposts) the durability is described as "11 9s". But, according to the AWS documentation, the S3 One Zone Infrequent Access (IA) does not reach the S3 standard and in fact the author point this because she tells us "Data in this class can be lost", but at the same time, the author argues that S3 One Zone Infrequent Access (IA) is 11 9s.
Description of S3 One Zone Infrequent Access(IA) https://aws.amazon.com/about-aws/whats-new/2018/04/announcing-s3-one-zone-infrequent-access-a-new-amazon-s3-storage-class/
On the other hand, this lesson is really great to have a global view of S3, I just think that this typo can lead to misunderstanding of the availability.
The AWS documentation you reference does state S3 One Zone-IA is 11 9’s in the line:
"Amazon S3 Standard, S3 Standard-IA, S3 One Zone-IA, and Amazon Glacier, are all designed for 99.999999999% durability"
https://aws.amazon.com/s3/storage-classes/ also states:
"Designed for durability of 99.999999999% of objects in a single Availability Zone"
With the caveat that "Because S3 One Zone-IA stores data in a single AWS Availability Zone, data stored in this storage class will be lost in the event of Availability Zone destruction".
Durability remains constant, but availability and resilience is less.