NFS access triggers conversion with ONTAP 4.0 or later, unless I'm
misreading the 5.3 docs.

"Data ONTAP 4.0", in the table under "Conversion to Unicode format", refers to the *directory format*, not to the OS the box is running.

What that table means is:

If your appliance is running *ONTAP 5.3[.x]*:

If the access is from CIFS, the directory being accessed is converted to Unicode format, regardless of what format it's in.

If the access is from NFS, the directory being accessed is converted to Unicode format if and only if it's in the "new" format introduced in ONTAP 4.0.

4.0 through 5.2[.x] only convert directories to that "new" format if the access comes from CIFS; NFS-only directories remain in the boring old format introduced by FASware 1.0, so the box don't spend resources generating 8.3 names and the like for directories not being accessed by Microsoft-flavored OSes.