Does Active Subscriber create contacts for external subscribers?

Yes. If a subscriber does not exist in Active Directory, Active Subscriber creates a new contact in Active Directory and stores it in the same organizational unit (OU) as the mailing lists.
N.B. Active Subscriber also deletes subscribers from the OU if it finds they are no longer members of any lists (or if it receives a non-delivery report), but it never creates or deletes contacts outside the Active Subscriber Data OU - it only modifies those contacts that it "owns".