Your point about revealing a POSIX that's actually there but hidden and thus visible to low-level Amazon employees building the S3 service and invisible to S3 end customers is true but that isn't the the point of the article. The author is saying there are motivations for a POSIX-like api to also be visible to the end user.
So your explanation of the "hidden POSIX" stack looks like 2 layers: POSIX api <-- AWS S3 built on top of that
Author's essay is actually talking about 3 layers: POSIX <-- AWS S3 <-- POSIX
That's why the blog post has the following links to POSIX-on-top-of-S3-objects :
https://github.com/s3fs-fuse/s3fs-fuse
It even supports GCS (as GCS has S3 compatible API)
https://github.com/s3fs-fuse/s3fs-fuse/wiki/Google-Cloud-Sto...
Later I used Panic's Transmit Disk but they removed the feature.
Recently I'd been looking at s3fs-fuse to use with gocryptfs but haven't actually installed it yet!