Actually, the size of an 'em' isn't even defined relative to the size of a pixel by the CSS spec, so the claim in the article is dead wrong about it helping you use em when you mean 10 pixels. 62.5% of an unknown value isn't 10 pixels.
It would also be very naive to argue that working in units of 10 is not much, much more convenient than working in units of 16.
The 'em' unit is equal to the computed value of the 'font-size'...
Therefore, em is in units of font size, and is not independent of it. It's totally unrelated to 'px' values, except by luck. Not all browsers default to 16px text. (And yes, it's true that px are actually defined as the distance subtended by an angle of 0.0227 degrees at a typical viewing distance of the device, but they're typically used interchangably with screen pixels)