No, Kodi would not pick up your credentials from fstab. If Kodi needed credentials and didn’t ask for them then you had previously entered them for that source and they got saved to ~/.kodi/userdata/passwords.xml
This would affect all paths where the source was using a network path which means that Kodi is doing the networking itself. It does not interact with the underlying OS so mount points are not affected.
You said you were using a system mount. From that log it seems your music library is configured using Kodi paths. You could try using max SMBv1, or you could test your mount point to see if it works better with that NAS. If the mount point works better you can use path substitution to reroute the path stored in your MySQL db to the local mount without affecting other clients.