Earlier quoted context omitted.
You're kinda both right. But the focus-protection part has nothing to do with SAS.
Consider https://superuser.com/a/61772
Nobody is denying that the "anti-hijacking" forcing of CTRL-ALT-Delete adds to security, what they're saying is that it has nothing to do with this topic.
This topic is about keyboard input focus. In Windows, due to the process hierarchy the login UI isn't running in the same context as desktop applications, so stealing focus or focus drift couldn't occur.