Is password complexity policy linked to Active Directory in SQL Server
As far as I know, as long as the users have been created with CHECK_POLICY they will inherit it from the domain.

If not, you'll have to enable the policy use for all of them.
Or use SQL Server 2012 BPA, as it has a check for this part (2).


  The security policy might be set in Windows, or might be received from the domain (1).


https://msdn.microsoft.com/en-us/library/ms161959(v=sql.110).aspx (1) 

https://support.microsoft.com/en-us/help/2028712/understanding-password-policy-for-sql-server-logins (2)