Also, until recently most browsers didn't support subpixel typographic adjustments (for instance, "letter-spacing: 0.4px"), and you could only adjust this stuff reliably if your font-size was higher than 20px.
That said, 16px is the default base font size on most user agents, and it seems adequate for reading on many devices. However, I tested 16px in Luis's site and it was not quite satisfactory. 19px seems to be the sweet spot in this case.
Ultimately, no font-size is perfect for everyone. Be thankful that your browser zoom works ;) Even on a desktop, sometimes at night, with f.lux on, I have to zoom HN to read it comfortably (thankfully Safari has tap to zoom in an area).