Well, it sure looks like a grain and is usually used like a grain, so what’s up? Fact is, quinoa is the seed of a plant in the goosefoot family, which is not a grass—and grains like wheat, rye, and corn are the seeds of grasses. Of course this technicality does not change the fact that it is tasty and nutritious, but it might be useful in a game of trivia!
I think for a change you’re being too pedantic rather than dispelling a myth. It depends if you’re talking Botanical classification, where grains are the seeds of cereals, or grasses, or Agronomy where the definition of “grains” includes what are called pseudocereals such as quinua, amaranth and buckwheat; pulses (i.e., the pea family including everything from peas to beans to lentils to peanuts); and oilseeds (e.g., mustards, sunflower, safflower, hemp, flax, poppy)