Can SQL Server OpenQuery/OpenRowSet to an LDAP Linked Server be used for Authentication?
I'm not sure why you'd need a linked server to LDAP to validate credentials? SQL Server has Active Directory (Windows) authentication built into it, so you should be able to leverage that as long as you add the appropriate Login object in SQL Server for either the AD User or AD Group that user is a part of, so they have the ability to connect.
Unless I'm missing something like you're trying to connect to LDAP of a different domain than that of the SQL Server instance, but then we'd more details on your goal.