كَمَا يُنْبِتُ الْمَاءُ الزَّرْعَ means “just as water causes crops to grow.” كَمَا introduces a comparison; الْمَاءُ is water, and الزَّرْعَ is crop or vegetation. The common point is the increase of an effect.
A comparison does not require every aspect of the two sides to be identical. Here the growth of vegetation is used to illuminate an inward moral danger, not to provide a literal measure of another person’s spiritual state.
Cultivating sincerity, honesty, and wholesome habits is a practical way to care for the heart.