Why is adding the greens and stems of Allium vineale late in the cooking process recommended?

To preserve their texture because the wilder greens can sometimes become slightly stringy.

When utilizing the long, green, strappy leaves and the tender white lower stems of onion grass in cooked dishes, such as savory batters or when incorporating them into hot meals, a specific technique regarding timing is suggested to maintain quality. These wilder greens have a propensity, unlike their cultivated counterparts, to develop a slightly stringy texture if subjected to prolonged heat. Therefore, foragers and cooks advise incorporating them only towards the very end of the cooking cycle, just before the dish is prepared for serving. This timing allows them to heat through and wilt slightly without losing too much structural integrity, thus preserving a more desirable mouthfeel.