What you're missing is that on a Mac, the Administrator account is not the highest level access. While I t's still limited, requiring name and password to do things and some things are still prohibited, the ROOT user is above Administrator level.
This vulnerability allows an admin to escalate his privileges by being able to open and write to ROOT-access-only files with impunity, regardless of what permissions are set on those files. That includes the files establishing who has access to what files, including ROOT files and who is a ROOT user! Privilege escalation from Admin to ROOT! Of course, on a Mac, the original Admin can create the first ROOT user and establish the ROOT password, so again, for most Mac users, this vulnerability is moot, because they could already do what it gives them the ability to do.
For some very limited number of Macs (I have trouble thinking of any, but it's possible) where an owner, who is the only one who knows the ROOT user name and password, has given admin privileges to one or two admins and some other users have only standard privileges, it might be a threat if one of the admins is too trustworthy. The it's an Oops!