The permissions on the parent and lock directory could restrict the access to a specific user and group, but yes, other processes could interfere with this locking if directed to do so.

One condition where this interference is helpful is a crash, where a @reboot entry in the crontab could:

  [ -d /your/lockdir ] && rmdir /your/lockdir

You would also not want to place the lock directory in /tmp or otherwise where other users could manipulate (or see) it. In Red Hat, there is a /var/run/lock directory that might be appropriate.

My biggest use case for directory locking in scripts is handling inotify events.