The problem is that HDFS ACLs protect the data in HDFS, but it does nothing to protect the metadata inside the Hive metastore. Sentry comes with a plugin for the Hive Metastore Server that is used for exactly that purpose.
You didn't clarify what you meant by AD. AD is a lot of things, and specifically for this conversation it could be the mechanism that provides group memberships, and/or it could be LDAP authentication for HiveServer2. Neither of these things are a substitute for Sentry, but rather, complementary pieces to integrate better with your enterprise infrastructure.