That's what the outer min() is for: it makes sure the font size caps out at 1.3em which usually translates to 16 x 1.3 = 20.8px, which is well within the recommended size range for prose anyway.
What that whole snippet does boils down to exactly what they said in the article:
> The main global stylesheet uses the browser default font size, smoothly scaled up to 130% on higher-resolution displays as the baseline for the body text of the whole document.
On a low-dpi screen, nothing changes. On a high-dpi one, if you haven't set your browser text size to something larger, this snippet saves you from tiny unreadable text. Also note that ctrl+ and ctrl- to zoom still work just fine. It's not as dramatic a change as the sibling comment said. You can try it out on their site to see for yourself.
