That comment is totally misguided. We do not want our data guaranteed to be on disk. Nobody said they wanted that. What we want is for the traditional unix way to save a file to (write to tempfile, rename over target) to *either* result in the old file or the new file. Not a zero byte file, losing both the old and the new data.
fsync is just the only way to work around this in the posix API, but its is much more heavy and gives much more guarantees than we want.