[maybe he happened to be in Yunmeng at some point! thirteen years is a long time and the older Wei Wuxian said the elder Lan Wangji likes to cook. there was plenty of time for him to pick it up incidentally and if he cooks for his Wei Wuxian frequently, it'd be natural for him to start there to teach his counterpart. yeah this all potentially makes sense, right?????

Lan Wangji is QUIETLY AND IMPASSIVELY TRIUMPHANT]