Generally when somebody speaks of a crop, they indeed talk about crops that end up as food. However, according to Merriam-Webster, a crop can refer to any plant or plant product that is grown by farmers. Tulips are indeed farmed and harvested, so even in English, this works. It really depends on the definition. It's most likely a gewas here to refer to the significance of the tulip as a product rather than it just being an important flower. The tulip is important to the Dutch economy, for example.