The Agile Platform only synchronizes users with the Active Directory (AD) if that is an explicit requirement. What the Agile Platform does when integrating with LDAP is that it uses the AD user credentials to log-in the application.
Furthermore, the Agile Platform needs to create a local user in the Platform Database so that the identity of the user in the AD can be related with the user inside the platform. This is simply a reference consistency mechanism. Only the username is stored locally and nothing else, unless the user information is explicitly synchronized.
You can check the Outsystems Evaluation Center for more information on Active Directory.