Deepseek just used FoundationDB to build a parallel filesystem. Parallel filesystem are a big deal -- their number, including proprietary ones, is probably in single digits.
The first one I encountered was DrFTPD circa 2004. But these days, any object storage system qualifies because they all support varying replication schemes and reading from any valid in-sync replica.
in most filesystems files are addressed by an inode. and directories are just lists of inodes. if you remove those then you end up with a KV store: inode->file.
consequently i see no difficulty to convert a KV store into a hierarchy by adding the necessary tables/directories representing the hierarchy.
In my book, a parallel filesystem is not just pooling together a bunch of nodes, but something that can actually support the synchronized accesses needed by a parallel workload. So not just decoupling between data and metadata, but scaling out of the metadata layer as well.
That and a hierarchical namespace (I could be sold on compromising some POSIX compliance for performance reasons, but it has to fundamentally be a hierarchical namespace with similar semantics). So object stores would not qualify.