Porter’s definition of strategy is normative, not descriptive. That is, it distinguishes a good strategy from a bad one. His focus is on content, not process. His focus is on where you want to be, not on the decision-making process by which you got there—not how, or even whether, you do formal strategic planning, nor whether your strategy can be captured in fifty words or less.
Strategy development is a design process. Porter’s work implies a lot of this with his focus on the ‘design’ of the resulting business model; it’s fit, coherence, focus, trade-offs. But he leaves open the question of ‘how is strategy created’ and this is where design theory is useful.
This is a gap in the strategy literature – being relatively underdeveloped and lacking a central figure in the same way Porter dominates ‘strategy as content’. If you move from Mintzbert/Liedtka/Martin (strategy development as design process) to Rumelt (strategy as diagnosis, guiding policy, coherent actions) to Porter (attributes of a good strategy in market) you have a pretty good foundation, but there is opportunity to build a much better understanding of the first area.