I came across the Tweet and it has raised a pretty good point on responsible disclosure, Apple's attitude towards support (and putting iOS first - e.g. making a bug bounty programme for iOS but none for macOS), and general 0-day shenanigans. See the hacker news thread for an in-depth but technical discussion.

Most important take-away: disabling root is not enough: if there's no root account (and/or you've disabled it) entering a new user root with a blank password will just create a root account. Or as your article mentions:

This makes four -- count them, four -- password-related security problems since High Sierra was released in September.

Which is why people (for example, in the hacker news thread) have a few speculations about what Apple's up to. Because this doesn't just mean if someone's logged in you can access anything locked by root account (e.g. everything private), but even if they're not logged in you're able to log in as a new user, and get access to pretty much everything!