That site doesn't claim that they aren't actually gluten free. What it does claim is that the data General Mills provided has not convinced the Canadian Celiac Association that the process for verifying that that there was no cross contamination was not rigorous enough. That is good info, but starting off with "Cheerios aren’t actually gluten free" is simply not true. My family has not had any issues due to any cross contamination with them (we would know immediately), but this does make me want to cut it out as the risk is simply too high. Thanks for the heads up.

Most of the nutritional value I've seen in cereals have been through supplemental ingredients. For example, a lot of the vitamins and minerals are added in as if you crushed supplements and put them in your bowl. There are better ways of getting those nutrients, including fiber: beans, chickpeas, lentils, peanuts, flax seeds, chia seeds, various vegetables, etc. None of those require milk to be palatable and they're much cheaper than the ultra-processed cereals. When it comes to fiber, you'll want a variety, cereals largely just rely on bran.