Hola Jenk

We are using the pluperfect in this sentence to express an action that "had happened". With "el ladrón había conseguido escapar" we are placing that action "before" the very first sentence in the dictation: "La policía no tardó mucho en llegar al bar." So, there are two actions: the first one happening is "the thief running away" [pluperfect], and the second action happening is "the police arriving". This is what the pluperfect is generally used for: to express an action in the past that takes place before another past action. 

Using the imperfect here wouldn't be good, as the nature of the meaning of "to manage to escape" will not trigger the imperfect which is generally descriptive. You could use the preterite though: "el ladrón consiguió escapar durante la confusión que hubo en ese momento". This way it gives the sense of a punctual and completed action.

Hope this helps. 

Saludos

Inma