Bonjour Tom,

You are correct that normally one would use Le Passé Composé. In this sentence, the action is seen in its duration as if the action is still ongoing. In French, it is called 'L'Imparfait historique' emphasising on the duration of the action and on the consequence of the action. It is usually accompanied by a precise expression of time (a date for example - like in the exercise).

I hope this is helpful.

Bonne journée !