In the article, Cecil writes, “The subtractive primaries are red, blue, and yellow–to be exact, magenta, cyan (light blue), and yellow.” To say that they are red, blue, and yellow is not merely imprecise, but flat wrong. If you are looking at the spectrum, cyan is as different from light blue as yellow is from either light red or light green. (See the article for how to mix light to get yellow and cyan.) The perceived similarity of cyan and light blue results from the fact that the human eye is rather poor at making this particular distinction. But to scientific instruments they are clearly different. “Light blue” is a name for an unsaturated blue, obtained (in the additive domain) by mixing some white light with pure blue. (If you started from maximum blue, you can add equal amounts of red and green and ‘borrow’ from the blue an equal amount to make the additive white.) The point is that a proper light blue will include some red (Indeed, as much red as green), and that color as a pigment for subtractive color mixing is not very useful. Cyan includes no red at all. The erroneous claimed equivalence of cyan and light blue is repeated several times (via appositive parentheticals). The implication that magenta and red are equivalent is equally suspect and more obviously so.
Sorry 'bout that.
On the other hand, though, you can’t exactly say that magenta, cyan, and yellow are the One True Set of subtractive primaries, either. There are many sets of colors you can use as a basis for the color space, and red, yellow, and blue comprise one such basis set, as any kindergartener could tell you. They might not span the color space quite as completely as CMY, but then, CMY doesn’t completely span the color space, either (part of the reason why most printing processes add a black pigment, instead of just mixing all three).
If you didn’t know what the words cyan or magenta mean, you might appreciate such parenthetical comments a little more. IOW, Cecil was not stating an equivalence.
Given the way it was presented, I still think it is reasonable to take it as an implied equivalence. If I were looking for a more familiar color name to suggest as being similar to cyan, I would try “turquoise”. In any case, the big difference is that cyan is a saturated color and light blue is not.