You might be mistaken because what you are quoting specifically talks about Same-desktop Elevation. While on Windows, UAC uses Secure Desktop by default, which is by definition a security boundary.
> You only have a security boundary in place if Windows asks you for a password when trying to run as Administrator.
Per the last sentence of the information that you quoted:
> By contrast, using Fast User Switching to sign in to a different session by using an administrator account involves a security boundary between the administrator account and the standard user session.
Fast User Switching requires the user to enter the administrator credentials in the UAC prompt.
