You can likely assume fsync is going on once you close a file writer. It's already a super expensive operation, so there would be little reason not to sync. The expensive part is they copy the entirety of the old file to a temp file, let you do changes to the temp file, then copy the temp file to the old file on close.

The implementation creates other problems for DBs though since you can't really do small writes efficiently. One idea I've had would be to implement a virtual paging system, but then you introduce a new layer of abstraction and it's still going to be really slow on NTFS (since it assumes a few large files, not many small ones).