The verb ‘usurper’ means to appropriate ourselves a right without any valid reason. In this case, this is a double negation where it actually means that the heroes have not wrongfully claimed their reputation. In other words, the sentence means that the heroes did not claim their reputation purely by themselves. “Je dois admettre que les soi-disant héros ne se sont pas totalement appropriés leur reputation.”

So you are right by saying that “I’ve got to admit that the so-called heroes have lived up to their reputation, after all … even if just a little bit.”

The verb “usurper” is not commonly used though. I would use “approprier” instead.

“Usurper” has the same meaning as the English “usurp”, but its usage is broader. In French, the verb can be applied not only to a specific title (“usurper le trône” = “usurp the throne”) but to many illegitimate claims to a positive quality, such as reputation. “X usurpe sa réputation” means that X’s reputation is undeserved. A straightforward translation of “ils n’ont pas totalement usurpé leur réputation” is “their reputation is not wholly undeserved”.