That this even gets discussed shows that we don’t know the difference between an issur and a minhag. 🙁
“Most of the farmers who grow oats for Cheerios also grow wheat and barley, which aren’t gluten free. Sometimes, those grains get mixed together in the fields or on the farm, so, to make Cheerios gluten free, we had to separate them.”
Wheat, barley, and oats are three of the Five Grains. They have nothing to do with kitniyot. The reason we Ashkenazim don’t eat kitniyot is because it is one of our longstanding minhagim, not because there is any issue of contamination of rice or beans with one of the Five Grains.