Thanks for clarifying: I was able to find the vulnerability on the unpatched system with:

  $ ls -al /
  (etc)
  -rw-r--r--   1 root  wheel       0 Aug  6 06:46   this_system_is_vulnerable

So I can a) confirm the vulnerability exists and it can write with root privileges.

and b) the patch works: I ran the patch, deleted the test file, rebooted and the file is no longer able to be written.