For this ...

    Any successful read() from each byte position in the file that  was
    modified  by  that  write  shall  return  the data specified by the write()

... perhaps I can re-open() and re-read() the same byte value written by another process for the same file, but the file contents may not have been fully flushed all the way to disk. The file contents may be "durable" across processes on the same running OS that mount the same filesystem ... but if the OS happens to die before the data is flushed, then perhaps after reboot the open()/read() will return an older value previously written.

The semantics of "durability" are a squishy concept.