No one, regardless of other issues, consider Quinoa to be a grain.
Some argue it is analagous to corn (maize as its called elsewhere), meaning it would be kitniyot. Other argues it is similar halachically to potatoes, which everyone considers to be non-kitniyot. The definition of kitniyot is very vague, but basically kitniyout on non-grains that are sufficiently similar to grains that there might be confusion. The “new world” products, introduced to Europe in the 1500s, have no ancient tradition, but we’ve settled that corn is, and potatoes aren’t, even though both can be used to make a grain-like product. Quinoa were only introducted to us recently.