That's not true. Dithering can be used in emissive screens, but dithering is not additive. If you mix red and green with color blending (e.g. by dithering), you get less red and less green in your mix, and therefore the resulting mix (a sort of ochre) is different from additive color mixing (yellow), where the amount of red and green stays the same. Or when you mix black and white, you get white with additive color mixing, but grey with blending. You also get grey when blending (dithering) red, green and blue. You can test this in software like Gimp, you won't be able to dither a full color image without at least the eight colors I mentioned.