That should be only for creating files, and maybe updating their metadata (not sure about that one).

The confusion stems from people thinking that files and directories are more different than they are. Both are inodes, and both are basically containers for data. File inodes are containers for actual data, while directory inodes are containers for other inodes.

All inodes need to be fsynced when you write to them. For files this is obviously when you write data to them. For directories, this is any time you change the inodes they contain, since you’re effectively writing data to them.

You only need to sync the direct parent because the containers aren’t transitive; the grandparent directory only stores a reference to the parent directory, not to the files within the parent directory. It’s basically a graph.