vIDM is only required for RBAC with NSX-T. If you want to use AD user accounts to access NSX-T Manager, you'll need vIDM. Otherwise, you can use the local admin account.
vIDM is needed to configure role-based access control for users (AD/LDAP users).
To enable the integration of NSX-T with vIDM, check the following: Configure VMware Identity Manager Integration
the key word is "required" so technically is not a dependency, so if you want to have RBAC on NSX then you required vIDM.