From the lesson «When talking about two actions that happen simultaneously, you will use :
en + Participe présent / Gérondif»; can you not also use imparfait and passé composé eg Je courait quand j'ai rencontré Mathilde ? I am not suggesting the same meaning or English translation, but the sentence still describes the simultaneous occurrence of events, that could also be described using le gérondif. If that is correct, the quote from the lesson should replace 'will use' with 'can use' (and preferably reference the lesson on passé composé and imparfait being used together as another). If there are reasons to choose one over the other, worth noting as well.