@Ereshkigal 

almost everytime for any customer LDAP needs to be connected through mid server as AD is in their internal network

check this

How to configure LDAP (step by step), and Troubleshoot information to resolve ldap cases. 

-> That's correct, you cannot authenticate via LDAP is mid server is used

-> along with user, you can import groups etc as well via mid server

LDAP is just source of user data, authentication will happen based on the credentials in ServiceNow, ldap don't store passwords.

LDAP is just used for authentication and when user logins to instance the system checks in AD.

The ServiceNow system does not support LDAP password authentication through a MID Server. A ServiceNow instance must be able to directly connect with an LDAP server to support password authentication.

If my response helped please mark it correct and close the thread so that it benefits future readers.