A more pedantic title would be "What's so bad about POSIX I/O for scalable performance on networked file systems". As others have said, it's an old problem that predates exascale by decades. I never asserted that POSIX is fundamentally broken; it's just that other communities with I/O issues at extreme scales decided to not even bother with the idea of POSIX and went straight for stateless, REST-like APIs and got atomicity through immutability.

It's a pronounced problem in high-performance computing because that community evolved from a much more primitive era and is dragging decades of legacy applications (and, to some degree, thinking) behind it.