S3 is fundamentally a key value store. The fact that you can view objects in “directories” is nothing more than a prefix filter. It is not a file system and has no concept of directories.
Indeed there’s every reason to believe that a modern file system would perform significantly faster if the hierarchy was implemented as a prefix filter than actually maintaining the hierarchical data structures (at least for most operations). You can guess that this might be the case that file creation is extremely slow on modern file systems (on the order of hundreds or maybe thousands per second on a modern NVME disk that can otherwise do millions of IOPs and listing the contents of an extremely large directory is exceedingly slow)
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).