No that’s incorrect. File size changes caused by append are covered by fdatasync in terms of durability guarantees.
It looks plausible: XFS's xfs_dio_write_end_io() updates the on-disk file size. Do you have a link to documentation that confirms this is true for Linux or POSIX filesystems?
Edit: POSIX 1003.1-2017 defines fdatasync(2) behavior in 3.384 Synchronized I/O Data Integrity Completion, where it says "For write, when the operation has been completed or diagnosed if unsuccessful. The write is complete only when the data specified in the write request is successfully transferred and all file system information required to retrieve the data is successfully transferred".
So I think POSIX does guarantee that a write at the end of the file with O_DSYNC/followed by fdatasync(2) (and therefore, Linux RWF_DSYNC) is sufficient. Thank you for pointing out that RWF_DSYNC is sufficient for appends, vlovich123!
RWF_DSYNC is sufficient and faster when data is overwritten without metadata changes to the file.