When using the kerberos authentication, the current windows user will negotiate the kerberos tokes to authenticate himself with Intel AMT device. Also this user needs to be added in the ACL in AMT along with the realm that can be accessed. If for some reason, kerberos connection cannot be established, the credentials entered as part of the username and password will be used. Once again, this set of username and password need to bepresent in the AMT device ACL list with the correct realms.
Also the AD integration guide says: Note that if the normal mode of operations is to connect using Kerberos, the username/password parameters to the library functions will be ignored.
Hope this helps.