Hola Estelle

In this specific exercise we are instructing in the hint to use the Imperfect, but if you're referring to a more general question as you could say both Lisa conoció / conocía un restaurante local bueno, we are using the imperfect here because this is more than anything background information as opposed to an action that had a beginning and and end. If we wanted to highlight in the text that she knew that restaurant "at some point" then you could say something like: "Lisa conoció un restaurante local hace unos días" - so this action is within that time frame, seen as something completed that happened that day. But what we express in the exercise with that sentence is not referring at a speficific time of knowing that restaurant, but it's just stating that she knew it (background information) - when that action of finding out about the restaurant is not relevant at all.

I hope this showed some light to your doubt.

Gracias y saludos

Inma