It doesn't have to be strict. Some people make too big of a deal out of it. Most of us are used to eating in certain combinations anyways and our bodies have adjusted. One small example that you've probably heard, turmeric's beneficial components are more bioavailable when eaten with black pepper. The body digests foods in varying ideal circumstances depending on what it is. Ultimately, you'll probably be just fine eating a varied diet, but there are combinations that are better or worse for nutrient uptake, gastric comfort, blood sugar spikes, etc.