It’s relevant, but also easy to measure. GI is a guess at what the product will do to your blood sugar. A diet high in fiber they included oatly with a meal is going to give a very different insulin response than oatly in isolation which will likely be different again when compared to pure maltose. I am not diabetic and had fun measuring my blood sugar for a while. Full meals with a lot of fiber, even if they included high GI foods didn’t do a lot to my blood sugar compared to high GI foods in isolation. So, as a diabetic, you just have to know how your body handles some foods and how best to consume them (or not).