That has nothing to do with this. Everyone can become root on their mac by typing their password. This exploit shows that you you can compile a bit of code that can then bypass that and run anything it wants with full root permissions.

In most cases, yes, but that's not what the parent meant. In managed environments, it's common to not have root login access, and local privilege escalation is sometimes more critical than normal.

Only if you have admin permissions. If you don’t have them, you need to type in the password of an admin to become root.