In context of the comment I was addressing, it’s clear that filesystem means more than just a key value store. I’d argue that this is generally true in common vernacular.
I think the previous poster could/should have said, "It is not a hierarchical file system and has no concept of directories." where I added the word "hierarchical".
But it's also pretty obvious that was the point.
For example, you could implement POSIX directory semantics on top of S3. About the only POSIX filesystem API you couldn’t implement it append / overwrite (well you could but it might be prohibitively expensive).