I'll use your pizza example; since I think we have similar thoughts but express them at two opposite extremes.
Orks do have genetic drives and knowledge, but I think its much more general, and you feel its more specific.
But, back to pizzas. Even with the innate ability to make a pizza, there will be some experimentation that will create non-pizzas. If you take the dough and add oil, diced tomato and herbs: you get a brushetta. Use only garlic with/without cheese and cut it into long thin slices you get garlic fingers. Fold the pizza over and seal it before cooking makes a calzone.
Your original gene-code didn't say make pizzas, garlic fingers, calzones, etc; just pizza.
Orks are like this; the gene-code isn't make gargants, trukks, kans, etc. Their genes allow them to make crude engines, hydrolics, combustable chemicals, etc. And they take the general info and make it into something useful. This usually takes the end forms of Gargants for the current generation of orks, but there is no really limit; and the fact that they display learning means they can continue expanding on ideas (like pizza dough to cake batter, opening whole new fields).