(sigh) You’re not paying attention. ;)
SHARING files is something entirely different than having them available via an iTunes Server. They are two totally separate, distinct facilities that have little to do with one another.
YES, you CAN sync files that are accessible via NAS devices. That’s how MY iTunes client is set up.
My wife uses the same NAS, but she’s using her OWN iTunes client.
But you CANNOT sync files that are offered to the iTunes Client via an iTunes Server. All the iTunes Client will do for content discovered via an iTunes Server is give you the opportunity to PLAY those files. They are NOT available for syncing via iTunes Server.
Can your NAS do BOTH? Absolutely. It can offer the SAME files through BOTH an iTunes Server connection OR via a CIFS/SAMBA “SHARE” connection.