Building an AD-compatible service is not that easy, and it was not easy circa 2006, because there was a lot to reverse engineer. Sure, Sun had access to its own directory service (Sun DS), MIT Kerberos, BIND, OpenLDAP, NSS, and Samba, so it had a lot of the pieces, but there was a lot of work to be done putting everything together.
For sure. But Sun had great engineers, and it was a feature that I'd argue was visibly existential at the time.