I got an alert from the CRC (Chicago Rabbinical Council) a recognized, widely accepted ORTHODOX hashgacha (for those that don’t know that) about this yesterday, as quoted in the first post above.
There is no reason quinoa should NOT be kosher l’Pesach as it is NOT a grain at all, nor was it ever designated in the kitniyos category.
As someone mentioned above, technically (though it looks a lot like bird seed when raw) it is related to the spinach family.