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.