S3 is obviously not a filesystem in the sense of a POSIX filesystem. And I would argue it is not a filesystem, even if we were to relax POSIX filesystem semantics (do not implement the full spec). But what is certainly possible is to span a filesystem on top of S3. It is basically possible to span a filesystem on anything that can store data. You can even go crazy for demonstration purposes and put a filesystem on top of YouTube (there are some tech demos for that on GitHub).
I think a better question is whether there are any good filesystem implementations on top of S3. There are many attempts like s3fs-fuse[^1] or seaweedfs[^2], but I have not heard many stories about their use at scale from big companies. Just recently there was a post here about cunoFS[^3]. It is a startup that implements a POSIX-compliant (supports symlinks, hard links (emulated), UIDs & GIDs, permissions, random writes, etc.) filesystem on top of S3/AZ/GCP storage and claims to have really good performance. I think only time will tell if it works out in practice for companies to use S3 as a filesystem through fs implementations on top of S3.
[^1]: https://github.com/s3fs-fuse/s3fs-fuse